Engineering Playbook — DSA

4Sum

Sat, 16 May 2026 00:00:00 GMT

Two nested anchors plus two-pointer inner scan — sort first, skip duplicates at all four levels.

01 Matrix

Sat, 16 May 2026 00:00:00 GMT

Distance from every cell to the nearest 0 — multi-source BFS.

3Sum

Sat, 16 May 2026 00:00:00 GMT

Find every unique triplet in an array summing to zero. Sort then two-pointer scan per anchor.

Add Binary

Sat, 16 May 2026 00:00:00 GMT

Schoolbook binary addition with carry — walk both strings from the right.

Accounts Merge

Sat, 16 May 2026 00:00:00 GMT

Merge user accounts sharing any email — Union-Find over emails.

Add Strings

Sat, 16 May 2026 00:00:00 GMT

Schoolbook addition from the rightmost digit — carry through, prepend remaining carry at the end.

Add to Array-Form of Integer

Sat, 16 May 2026 00:00:00 GMT

Add k digit-by-digit from the LSB end, mixing in k's digits via carry.

Add Two Numbers

Sat, 16 May 2026 00:00:00 GMT

Walk both linked lists in parallel, summing with carry; allocate one output node per step.

Adding Two Negabinary Numbers

Sat, 16 May 2026 00:00:00 GMT

Add base -2 numbers digit-by-digit — borrow propagates differently; trim leading zeros.

Additive Number

Sat, 16 May 2026 00:00:00 GMT

Is the string a Fibonacci-like additive sequence? Backtrack over the first two summands.

Alien Dictionary

Sat, 16 May 2026 00:00:00 GMT

Determine the alien character order from a list of sorted alien words — topological sort over derived precedence.

All O`one Data Structures

Sat, 16 May 2026 00:00:00 GMT

O(1) inc/dec/getMaxKey/getMinKey — doubly-linked list of count buckets, each holding the keys at that count.

All Paths From Source to Target

Sat, 16 May 2026 00:00:00 GMT

All paths from node 0 to n-1 in a DAG — DFS emitting paths.

Append Characters to String to Make Subsequence

Sat, 16 May 2026 00:00:00 GMT

Find the minimum number of characters to append to s so that t becomes a subsequence — count unmatched suffix of t.

Assign Cookies

Sat, 16 May 2026 00:00:00 GMT

Maximum number of content children given sorted greed factors and cookie sizes — two-pointer greedy.

Asteroid Collision

Sat, 16 May 2026 00:00:00 GMT

Stack — push each asteroid; on a right-moving stack top vs left-moving incoming, pop the smaller (or both if equal).

Average of Levels in Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Return the average value at each depth of a binary tree.

Balanced Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Recursion returns height — propagate a -1 sentinel up if any subtree is unbalanced.

Basic Calculator

Sat, 16 May 2026 00:00:00 GMT

Evaluate expressions with +, -, and parentheses by tracking a running sign through a stack of suspended sums.

Before and After Puzzle

Sat, 16 May 2026 00:00:00 GMT

Concatenate phrases sharing a boundary word — group by first word and last word.

Best Meeting Point

Sat, 16 May 2026 00:00:00 GMT

Manhattan distance separates into 1D medians on rows and columns — sum each axis's deviations from the median.

Best Time to Buy and Sell Stock II

Sat, 16 May 2026 00:00:00 GMT

Multiple buy/sell transactions, capture every positive day-over-day delta.

Best Time to Buy and Sell Stock III

Sat, 16 May 2026 00:00:00 GMT

At most two transactions — track four running states (after each buy/sell).

Binary Search

Sat, 16 May 2026 00:00:00 GMT

Classic binary search on a sorted array — the atomic primitive.

Best Time to Buy and Sell Stock

Sat, 16 May 2026 00:00:00 GMT

Single buy/sell transaction maximizing profit — sweep tracking running minimum.

Binary Subarrays With Sum

Sat, 16 May 2026 00:00:00 GMT

Count 0/1 subarrays summing to goal — atMost(goal) − atMost(goal−1).

Binary Tree Cameras

Sat, 16 May 2026 00:00:00 GMT

Minimum cameras to monitor all nodes — greedy bottom-up with three states (covered, has camera, uncovered).

Binary Tree Maximum Path Sum

Sat, 16 May 2026 00:00:00 GMT

Post-order DFS returns the best single-arm gain; meanwhile track the best path bending at any node.

Level Order Traversal of Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Return node values grouped per depth using a queue-based BFS.

Binary Tree Paths

Sat, 16 May 2026 00:00:00 GMT

All root-to-leaf paths in a binary tree — DFS with path-building.

Binary Tree Right Side View

Sat, 16 May 2026 00:00:00 GMT

Preorder DFS visiting right before left — first node seen at each depth is the rightmost.

Binary Watch

Sat, 16 May 2026 00:00:00 GMT

Enumerate all hh:mm times whose binary representations have exactly `turnedOn` LEDs.

Binary Tree Preorder Traversal

Sat, 16 May 2026 00:00:00 GMT

Iterative stack — push right child first so left is processed next.

Binary Tree Zigzag Level Order Traversal

Sat, 16 May 2026 00:00:00 GMT

BFS level order with alternating direction per depth — reverse on odd levels.

Build a Matrix with Conditions

Sat, 16 May 2026 00:00:00 GMT

Place [1..k] in a k×k grid satisfying row and column ordering constraints — two independent topological sorts.

Boats to Save People

Sat, 16 May 2026 00:00:00 GMT

Minimum boats to ferry people with weight limit — sort, pair heaviest with lightest if they fit.

Burst Balloons

Sat, 16 May 2026 00:00:00 GMT

Maximize coins from bursting balloons — interval DP keyed by the LAST balloon burst in each range.

Bulls and Cows

Sat, 16 May 2026 00:00:00 GMT

Compute bulls (correct digit + position) and cows (correct digit, wrong position) for a guess.

Bulb Switcher

Sat, 16 May 2026 00:00:00 GMT

Bulb i ends on iff i has an odd number of divisors — iff i is a perfect square. Answer is floor(sqrt(n)).

Bus Routes

Sat, 16 May 2026 00:00:00 GMT

BFS on the bipartite stop–route graph — each layer represents one bus transfer.

Can Place Flowers

Sat, 16 May 2026 00:00:00 GMT

Greedy single-pass — plant a flower whenever current, prev, and next are all 0.

Candy

Sat, 16 May 2026 00:00:00 GMT

Each child gets at least one candy; higher-rated child gets more than smaller-rated neighbors. Minimum candies via two passes.

Car Pooling

Sat, 16 May 2026 00:00:00 GMT

Determine if all pickups can fit under capacity using a delta-array sweep on locations.

Check If a Word is a Prefix of Any Word in a Sentence

Sat, 16 May 2026 00:00:00 GMT

Return the 1-indexed position of the first sentence word that starts with `searchWord`, or -1.

Check If It Is a Straight Line

Sat, 16 May 2026 00:00:00 GMT

All points lie on one line iff every point shares the cross-product direction with the first edge.

Check if One String Swap Can Make Strings Equal

Sat, 16 May 2026 00:00:00 GMT

Can a single character swap on one string make the two equal? Count mismatch positions.

Cheapest Flights Within K Stops

Sat, 16 May 2026 00:00:00 GMT

Bellman-Ford limited to k+1 edges — relax k+1 times.

Cherry Pickup

Sat, 16 May 2026 00:00:00 GMT

Max cherries on two paths from top-left to bottom-right then back — simulate two simultaneous downward paths in 3D DP.

Circular Array Loop

Sat, 16 May 2026 00:00:00 GMT

Detect a cycle of length > 1 that flows in a single direction using fast/slow pointers per start position.

Climbing Stairs

Sat, 16 May 2026 00:00:00 GMT

Distinct ways to climb n stairs with 1- or 2-step moves — Fibonacci.

Closest Node to Path in Tree

Sat, 16 May 2026 00:00:00 GMT

For each query, find the node on the path between two endpoints that is closest to a target node.

Clone Graph

Sat, 16 May 2026 00:00:00 GMT

BFS or DFS with a hash map from original node to clone — copy nodes lazily, link neighbors as discovered.

Coin Change

Sat, 16 May 2026 00:00:00 GMT

Fewest coins summing to amount — bottom-up unbounded knapsack.

Combination Sum

Sat, 16 May 2026 00:00:00 GMT

All multisets of given candidates summing to target — DFS with start-index for dedup.

Combinations

Sat, 16 May 2026 00:00:00 GMT

All k-combinations of [1..n] — DFS with start-index to avoid duplicates.

Compilation Order

Sat, 16 May 2026 00:00:00 GMT

curriculum-track — return a valid build order for projects given their dependencies via topological sort.

Complement of Base 10 Integer

Sat, 16 May 2026 00:00:00 GMT

Flip every significant bit — XOR with a mask of "all ones up to the MSB".

Confusing Number

Sat, 16 May 2026 00:00:00 GMT

Map each digit to its 180-degree rotation; reject digits that have no rotation; check the rotated value differs from original.

Connect All Siblings of a Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Wire every node's `next` pointer to the next node in level-order across the entire tree.

Build Binary Tree from Preorder and Inorder Traversal

Sat, 16 May 2026 00:00:00 GMT

Preorder gives the next root; index lookup in inorder splits left and right subtree ranges.

Contains Duplicate II

Sat, 16 May 2026 00:00:00 GMT

Detect any duplicate pair within index distance k using a sliding hash set.

Container with Most Water

Sat, 16 May 2026 00:00:00 GMT

Two pointers — at each step the shorter wall is the bottleneck, so move it inward.

Construct Target Array with Multiple Sums

Sat, 16 May 2026 00:00:00 GMT

Reverse-engineer the build from target back to all-ones using a max-heap and modular replacement.

Contains Duplicate

Sat, 16 May 2026 00:00:00 GMT

Hash set membership test — return true on the first repeat.

Contiguous Array

Sat, 16 May 2026 00:00:00 GMT

Longest contiguous subarray with equal numbers of 0s and 1s — prefix sum with `+1/-1` encoding.

Convert 1D Array Into 2D Array

Sat, 16 May 2026 00:00:00 GMT

Validate dimensions match the input length, then copy contiguous slices via indexing.

Continuous Subarray Sum

Sat, 16 May 2026 00:00:00 GMT

Does any subarray of length >= 2 sum to a multiple of k? Prefix sums modulo k.

Convert Sorted Array to Binary Search Tree

Sat, 16 May 2026 00:00:00 GMT

Recursively pick the middle as root — both halves become left and right subtrees.

Count Anagrams

Sat, 16 May 2026 00:00:00 GMT

Count distinct anagrams of every space-separated word, multiplied together modulo 1e9+7.

Convex Polygon

Sat, 16 May 2026 00:00:00 GMT

A polygon is convex iff all consecutive edge cross products have the same sign.

Count and Say

Sat, 16 May 2026 00:00:00 GMT

Generate the n-th term of the look-and-say sequence by repeatedly run-length encoding the previous term.

Count Days Without Meetings

Sat, 16 May 2026 00:00:00 GMT

Number of days in [1, n] free of any meeting — merge intervals, count gaps.

Count Pairs Whose Sum is Less than Target

Sat, 16 May 2026 00:00:00 GMT

Count pairs (i, j) with i < j and nums[i] + nums[j] < target — sort then converging two pointers.

Count Negative Numbers in a Sorted Matrix

Sat, 16 May 2026 00:00:00 GMT

Staircase walk from the bottom-left, counting an entire row tail or stepping up.

Count Pairs in Two Arrays

Sat, 16 May 2026 00:00:00 GMT

Transform to nums1[i] - nums2[i], sort, and count pairs whose sum exceeds zero with two pointers.

Count Substrings With K-Frequency Characters II

Sat, 16 May 2026 00:00:00 GMT

Count substrings of s where at least one character appears at least k times — atLeast counting via sliding window.

Count Subarrays With Score Less Than K

Sat, 16 May 2026 00:00:00 GMT

Count subarrays where (sum × length) < k, using a sliding window over positive integers.

Count Subarrays With Fixed Bounds

Sat, 16 May 2026 00:00:00 GMT

Count subarrays whose min equals minK and max equals maxK using a sliding-window with three trackers.

Count Substrings with Only One Distinct Letter

Sat, 16 May 2026 00:00:00 GMT

Sum of triangular numbers — each run of length k contributes k*(k+1)/2 single-letter substrings.

Count the Number of Good Subsequences

Sat, 16 May 2026 00:00:00 GMT

Count subsequences where every distinct character appears the same number of times — combinatorial sum over candidate frequency.

Count Triplets That Can Form Two Arrays of Equal XOR

Sat, 16 May 2026 00:00:00 GMT

Reduces to counting (i, k) pairs with prefix XOR equal — sum (k - i) for each such pair.

Counting Bits

Sat, 16 May 2026 00:00:00 GMT

Bit count of every i in [0, n] in O(n) using dp[i] = dp[i >> 1] + (i & 1).

Course Schedule II

Sat, 16 May 2026 00:00:00 GMT

Return a valid course order — Kahn's BFS topological sort.

Course Schedule

Sat, 16 May 2026 00:00:00 GMT

Can all courses be finished given prerequisites? Cycle detection via Kahn's BFS topological sort.

Create Maximum Number

Sat, 16 May 2026 00:00:00 GMT

From two digit arrays, pick subsequences of lengths summing to k and merge them into the lexicographically largest k-digit number.

Cyclic Sort

Sat, 16 May 2026 00:00:00 GMT

curriculum problem — sort `nums` containing exactly the integers [1..n] in O(n) via in-place swap.

Custom Sort String

Sat, 16 May 2026 00:00:00 GMT

Reorder `s` so its characters follow the custom alphabet `order` first.

Data Stream as Disjoint Intervals

Sat, 16 May 2026 00:00:00 GMT

Maintain disjoint sorted intervals under add(int) and getIntervals() via a std::map keyed by start.

Daily Temperatures

Sat, 16 May 2026 00:00:00 GMT

Monotonic decreasing stack — pop indices whose temperature is exceeded and record the gap.

Decode String

Sat, 16 May 2026 00:00:00 GMT

Stacks of (count, prefix); on each `]` apply the multiplication and concatenate back to the outer prefix.

Decode Ways

Sat, 16 May 2026 00:00:00 GMT

Number of decodings of a digit string under the A=1, B=2, ..., Z=26 mapping — 1D DP.

Delete N Nodes After M Nodes of a Linked List

Sat, 16 May 2026 00:00:00 GMT

curriculum-variant — keep m, delete n, repeat throughout the list.

Delete Nodes And Return Forest

Sat, 16 May 2026 00:00:00 GMT

Post-order DFS — a node whose value is to delete returns null and pushes its non-null children as roots.

Design HashMap

Sat, 16 May 2026 00:00:00 GMT

Implement a hash map from scratch using array buckets with chaining.

Design Add and Search Words Data Structure

Sat, 16 May 2026 00:00:00 GMT

Trie with wildcard `.` search via DFS branching on unknown characters.

Design File System

Sat, 16 May 2026 00:00:00 GMT

Hash map of full path to value — createPath fails if path exists or parent missing.

Design In-Memory File System

Sat, 16 May 2026 00:00:00 GMT

Trie of directories where leaf nodes carry a content string — recursive path walk for ls/mkdir/read/write.

Design HashSet

Sat, 16 May 2026 00:00:00 GMT

Open-addressing or separate-chaining hash set — bucket by hash, linear list inside each bucket.

Design Tic-Tac-Toe

Sat, 16 May 2026 00:00:00 GMT

Detect a winner in O(1) per move by tracking row/column/diagonal sums.

Detonate the Maximum Bombs

Sat, 16 May 2026 00:00:00 GMT

Build edge i to j iff bomb i reaches bomb j; BFS/DFS from each source and take the max reachable.

Diet Plan Performance

Sat, 16 May 2026 00:00:00 GMT

Score a length-k window by total calories vs lower/upper limits — fixed-size sliding window.

Diameter of Binary Tree

Sat, 16 May 2026 00:00:00 GMT

DFS returns subtree depth; meanwhile track best left + right pair at any bend.

Distinct Subsequences

Sat, 16 May 2026 00:00:00 GMT

Count distinct subsequences of s that equal t — 2D DP over prefix counts.

Divide Array Into Increasing Sequences

Sat, 16 May 2026 00:00:00 GMT

Can we partition a sorted array into ≥k-length strictly increasing subsequences? Max frequency check.

Divide Chocolate

Sat, 16 May 2026 00:00:00 GMT

Cut the chocolate bar into k+1 contiguous pieces maximizing the minimum piece sum — binary search on the answer.

Dot Product of Two Sparse Vectors

Sat, 16 May 2026 00:00:00 GMT

Implement dot product for vectors with mostly zero entries — store only non-zero indices.

Employee Free Time

Sat, 16 May 2026 00:00:00 GMT

Find common free intervals across employee schedules — merge all intervals, take the gaps.

Encode and Decode Strings

Sat, 16 May 2026 00:00:00 GMT

Length-prefix framing — encode each string as "len#payload"; decode by parsing the length up to the delimiter, then slicing.

Equal Rational Numbers

Sat, 16 May 2026 00:00:00 GMT

Parse non-repeating + repeating parts, expand to a long decimal, compare floats with tolerance — or reduce to fractions.

Dungeon Game

Sat, 16 May 2026 00:00:00 GMT

Minimum starting HP to reach the bottom-right alive — DP from bottom-right backwards.

Evaluate Reverse Polish Notation

Sat, 16 May 2026 00:00:00 GMT

Stack of operands — on operator, pop two, apply, push result.

Erect the Fence

Sat, 16 May 2026 00:00:00 GMT

Convex hull via Andrew's monotone chain — return all hull points including collinear edge points.

Evaluate Division

Sat, 16 May 2026 00:00:00 GMT

Compute `a / d` from given pairwise ratios — DFS over the ratio graph (or weighted Union-Find).

Exclusive Time of Functions

Sat, 16 May 2026 00:00:00 GMT

Stack of currently-running functions; on each log line, charge elapsed time to the stack top.

Fibonacci Number

Sat, 16 May 2026 00:00:00 GMT

Iterative two-variable rolling — O(1) space.

Find All Anagrams in a String

Sat, 16 May 2026 00:00:00 GMT

Return all start indices of `p`'s anagrams in `s` — sliding window of letter counts.

Final Array State After K Multiplication Operations I

Sat, 16 May 2026 00:00:00 GMT

Apply k multiplications — each picks the smallest element (smallest index on ties) and multiplies it by m.

Find All Duplicates in an Array

Sat, 16 May 2026 00:00:00 GMT

Same sign-flip trick — if `nums[|v| - 1]` is already negative, `|v|` is a duplicate.

Find All Numbers Disappeared in an Array

Sat, 16 May 2026 00:00:00 GMT

Use the array itself as a hash — negate the value at index `|nums[i]| - 1`; positives left are the missing.

Find All Possible Recipes from Given Supplies

Sat, 16 May 2026 00:00:00 GMT

Which recipes are makeable from initial supplies? Topological sort over the ingredient DAG.

Find Center of Star Graph

Sat, 16 May 2026 00:00:00 GMT

The center node appears in every edge — so it must appear in the first two edges.

Find Duplicate File in System

Sat, 16 May 2026 00:00:00 GMT

Group files by content using a hash map keyed on the content blob.

Find First and Last Position of Element in Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Return the [first, last] indices of target via two boundary binary searches (lower_bound and upper_bound).

Find if Path Exists in Graph

Sat, 16 May 2026 00:00:00 GMT

BFS/DFS or Union-Find to check connectivity between two nodes in an undirected graph.

Find K Closest Elements

Sat, 16 May 2026 00:00:00 GMT

Binary search on the leftmost index of the k-sized window minimizing distance to x.

Find K Pairs with Smallest Sums

Sat, 16 May 2026 00:00:00 GMT

Among all pairs (a, b) from two sorted arrays, return the k with the smallest sum — heap over the implicit grid.

Find K-Sum Subsets

Sat, 16 May 2026 00:00:00 GMT

curriculum variant — return the k subsets with the largest sums, via heap-based BFS over subset construction.

Find K-th Smallest Pair Distance

Sat, 16 May 2026 00:00:00 GMT

Binary search the distance; count pairs with distance at most mid using a sorted two-pointer sweep.

Find Longest Self-Contained Substring

Sat, 16 May 2026 00:00:00 GMT

Find the longest substring such that no letter inside it appears anywhere outside the substring.

Find Median from a Data Stream

Sat, 16 May 2026 00:00:00 GMT

Maintain the running median with two heaps — max-heap for the lower half, min-heap for the upper half.

Find Minimum Diameter After Merging Two Trees

Sat, 16 May 2026 00:00:00 GMT

Connect two trees with one edge minimizing the resulting tree's diameter.

Find Peak Element

Sat, 16 May 2026 00:00:00 GMT

Find any peak (greater than both neighbors) in O(log n) using binary search on the slope.

Find Minimum in Rotated Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Binary search — compare mid to right end to decide which half holds the rotation pivot.

Find Minimum in Rotated Sorted Array II

Sat, 16 May 2026 00:00:00 GMT

Find the minimum in a rotated sorted array that may contain duplicates — binary search with shrink-on-tie.

Find Subsequence of Length K with the Largest Sum

Sat, 16 May 2026 00:00:00 GMT

Pick the k largest values then output them in original-order.

Find Right Interval

Sat, 16 May 2026 00:00:00 GMT

For each interval, find the index of the interval whose start is the smallest value ≥ this interval's end — sorted array + binary search.

Find the Difference

Sat, 16 May 2026 00:00:00 GMT

XOR all characters of both strings — the lone unmatched char survives.

Find Target Indices After Sorting Array

Sat, 16 May 2026 00:00:00 GMT

Without sorting, count smaller and equal elements to derive the target's index range.

Find The Duplicate Number

Sat, 16 May 2026 00:00:00 GMT

Find the duplicate in an array of n+1 integers in range [1, n] using Floyd's cycle detection on indices.

Find the Distance Value Between Two Arrays

Sat, 16 May 2026 00:00:00 GMT

For each element of arr1, binary search arr2 for the nearest neighbor and check the gap.

Find the K-Sum of an Array

Sat, 16 May 2026 00:00:00 GMT

kth largest of all subsequence sums — flip signs of negatives, sort, then BFS through pop-or-replace via a min-heap.

Find the Kth Largest Integer in the Array

Sat, 16 May 2026 00:00:00 GMT

kth-largest where elements are strings of arbitrary length — custom comparator on length then lex.

Find the First K Missing Positive Numbers

Sat, 16 May 2026 00:00:00 GMT

curriculum-track — return the first k positive integers missing from `nums` using cyclic sort then post-scan.

Find The K-th Lucky Number

Sat, 16 May 2026 00:00:00 GMT

Lucky numbers (digits in {6,8}) enumerated in order are binary numerals over the digit pair — convert k+1 to binary, map bits.

Find the Longest Substring Having Vowels in Even Counts

Sat, 16 May 2026 00:00:00 GMT

Bitmask parity of vowel counts as state — first-occurrence map gives the longest range with all-zero parity.

Find the Town Judge

Sat, 16 May 2026 00:00:00 GMT

A judge has in-degree n-1 and out-degree 0 — one pass through trust counts both.

Find Words That Can Be Formed by Characters

Sat, 16 May 2026 00:00:00 GMT

Sum lengths of words whose characters are a multiset subset of an inventory string.

Find the Lexicographically Largest String From Box II

Sat, 16 May 2026 00:00:00 GMT

curriculum-curriculum variant — given a string and a per-step swap budget, produce the lexicographically largest result via greedy two-pointer swaps.

Finding MK Average

Sat, 16 May 2026 00:00:00 GMT

MK-average over a stream — sliding window of last m, three multisets (low/mid/high) for trimmed-mean maintenance.

First Bad Version

Sat, 16 May 2026 00:00:00 GMT

Find the first index where isBadVersion(i) becomes true — boundary binary search.

First Missing Positive

Sat, 16 May 2026 00:00:00 GMT

Smallest missing positive integer — cyclic sort placing each value at index value-1.

First Unique Character in a String

Sat, 16 May 2026 00:00:00 GMT

Return the index of the first non-repeating character, or -1.

Flatten Binary Tree to Linked List

Sat, 16 May 2026 00:00:00 GMT

Right-root-left reverse traversal — append each node to the front of a growing chain.

Flatten Nested List Iterator

Sat, 16 May 2026 00:00:00 GMT

Stack of reverse-iterators; unwind lists lazily so each next() is amortized O(1).

Flip Columns For Maximum Number of Equal Rows

Sat, 16 May 2026 00:00:00 GMT

Two rows become identical after some column flips iff they're equal or exact complements; bucket-count both forms.

Flipping an Image

Sat, 16 May 2026 00:00:00 GMT

Reverse each row, then invert each bit — fuse both ops by walking from both ends inward.

Flip Game

Sat, 16 May 2026 00:00:00 GMT

All next states after flipping any consecutive "++" to "--" — linear scan.

Fraction to Recurring Decimal

Sat, 16 May 2026 00:00:00 GMT

Long-divide numerator by denominator and wrap the repeating cycle in parentheses.

Freedom Trail

Sat, 16 May 2026 00:00:00 GMT

Minimum steps to spell `key` by rotating a ring — DP over (key index, ring position).

Flood Fill

Sat, 16 May 2026 00:00:00 GMT

Recolor a connected region in a grid — DFS or BFS from the start cell.

Frequency of the Most Frequent Element

Sat, 16 May 2026 00:00:00 GMT

After sorting, slide a window while the cost of raising all elements to the max ≤ k.

Frog Jump

Sat, 16 May 2026 00:00:00 GMT

Can a frog reach the last stone with jump-size constraints? Hash-map per-stone reachable jumps.

Game of Life

Sat, 16 May 2026 00:00:00 GMT

Encode next-state in the high bit so each cell carries (current, next) without extra storage.

Frog Position After T Seconds

Sat, 16 May 2026 00:00:00 GMT

Probability that a frog starting at node 1 lands on target after exactly T seconds — BFS with branching probabilities.

Gas Station

Sat, 16 May 2026 00:00:00 GMT

Find a starting gas station so you can complete a circular tour — single-pass greedy with a balance reset.

Fruit Into Baskets

Sat, 16 May 2026 00:00:00 GMT

Longest subarray with at most two distinct values — variable window with a small frequency map.

Get the Maximum Score

Sat, 16 May 2026 00:00:00 GMT

Walk two sorted arrays in lockstep; at each shared element take the better of the two intermediate sums.

Generate Parentheses

Sat, 16 May 2026 00:00:00 GMT

Generate all well-formed parentheses combinations of length 2n — DFS with open/close counters.

Greatest Common Divisor of Strings

Sat, 16 May 2026 00:00:00 GMT

Strings have a divisor iff str1 + str2 == str2 + str1; if so, the answer is the gcd-length prefix.

Graph Valid Tree

Sat, 16 May 2026 00:00:00 GMT

A tree on n nodes has exactly n-1 edges and is connected; union-find or BFS verifies both.

Group Anagrams

Sat, 16 May 2026 00:00:00 GMT

Group strings that are anagrams of each other — hash map keyed by sorted form.

H-Index

Sat, 16 May 2026 00:00:00 GMT

Bucket counting on citation counts to find the maximal h with at least h papers cited h or more times.

Hamming Distance

Sat, 16 May 2026 00:00:00 GMT

XOR the two numbers, then popcount the differing bits.

Happy Number

Sat, 16 May 2026 00:00:00 GMT

Detect happiness (square-digit-sum converges to 1) vs cycle via Floyd's two-pointer technique.

Height of Binary Tree After Subtree Removal Queries

Sat, 16 May 2026 00:00:00 GMT

Precompute per-node depth and best sibling height; for each query return the appropriate maximum.

High Five

Sat, 16 May 2026 00:00:00 GMT

For each student, return the average of their top 5 scores — group then partial sort.

House Robber III

Sat, 16 May 2026 00:00:00 GMT

Tree-shaped house arrangement — return max loot using DP returning (rob-root, skip-root) per node.

House Robber II

Sat, 16 May 2026 00:00:00 GMT

Houses arranged in a circle — run linear House Robber on two ranges, exclude either head or tail.

House Robber

Sat, 16 May 2026 00:00:00 GMT

Rolling DP — at each house, choose max of "skip" or "rob + value two back".

Implement Queue Using Stacks

Sat, 16 May 2026 00:00:00 GMT

Two stacks — push onto in-stack; pop/peek lazily transfer to out-stack to reverse order.

Implement LRU Cache

Sat, 16 May 2026 00:00:00 GMT

O(1) get and put — hash map keyed to a doubly-linked-list node, splice node to the front on access, evict the tail on overflow.

Index Pairs of a String

Sat, 16 May 2026 00:00:00 GMT

Return all (start, end) index pairs where the substring exists in the dictionary — trie walk from each start.

Implement Trie

Sat, 16 May 2026 00:00:00 GMT

Build a prefix tree supporting insert, search, and startsWith in O(L).

Insert Delete GetRandom O(1)

Sat, 16 May 2026 00:00:00 GMT

Hash map of value to index plus a dense vector — swap-with-last for O(1) delete, uniform random by index.

Inorder Successor in BST

Sat, 16 May 2026 00:00:00 GMT

Walk the BST; whenever current value exceeds p, record it as a candidate and descend left.

Insert Interval

Sat, 16 May 2026 00:00:00 GMT

Insert a new interval into a sorted non-overlapping list, merging as needed — single linear pass.

Insert into a Sorted Circular Linked List

Sat, 16 May 2026 00:00:00 GMT

Insert a value into a sorted circular list at the correct position, handling wrap-around and uniform-value cases.

Integer to English Words

Sat, 16 May 2026 00:00:00 GMT

Recurse over thousands groups — each group of 3 digits converted to "hundreds tens units", suffixed by "Thousand"/"Million"/"Billion".

Interleaving String

Sat, 16 May 2026 00:00:00 GMT

Is s3 an interleaving of s1 and s2? 2D DP over consumed prefixes.

Intersection of Two Arrays II

Sat, 16 May 2026 00:00:00 GMT

Multiset intersection — match each element with multiplicity using a counter.

Intersection of Two Linked Lists

Sat, 16 May 2026 00:00:00 GMT

Find the node where two singly-linked lists merge, in O(1) extra space, via the pointer-swap technique.

Intersection of Two Arrays

Sat, 16 May 2026 00:00:00 GMT

Return the unique elements common to both arrays — hash set lookup.

Interval List Intersections

Sat, 16 May 2026 00:00:00 GMT

Intersect two lists of disjoint sorted intervals — merge-walk with two pointers.

Invert Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Recursively swap each node's children — a four-line classic.

IPO (Maximize Capital)

Sat, 16 May 2026 00:00:00 GMT

Pick up to k profitable projects with starting capital — greedy with a min-heap of affordable projects and a max-heap of profits.

Is Subsequence

Sat, 16 May 2026 00:00:00 GMT

Determine if s is a subsequence of t using a single greedy two-pointer walk.

Island Perimeter

Sat, 16 May 2026 00:00:00 GMT

Each land cell contributes 4 minus 2 per shared edge with a neighboring land cell.

Isomorphic Strings

Sat, 16 May 2026 00:00:00 GMT

Two strings are isomorphic iff there is a bijection between their characters preserving order.

Jewels and Stones

Sat, 16 May 2026 00:00:00 GMT

Count stones whose type appears among the jewels — hash-set membership.

Jump Game II

Sat, 16 May 2026 00:00:00 GMT

Minimum jumps to reach the last index — BFS-style "current reach" frontier.

K Closest Points to Origin

Sat, 16 May 2026 00:00:00 GMT

Return the k closest points to the origin using a max-heap of size k.

Jump Game I

Sat, 16 May 2026 00:00:00 GMT

Can you reach the last index? Track the maximum reachable index in one greedy pass.

K Empty Slots

Sat, 16 May 2026 00:00:00 GMT

First day on which two bloomed flowers are exactly k slots apart with all between unbloomed — sliding-window min over a day-array.

K Maximum Sum Combinations From Two Arrays

Sat, 16 May 2026 00:00:00 GMT

curriculum-track — k largest sums of a[i] + b[j], explored from (sorted top corner) outward via a max-heap.

K-th Smallest in Lexicographical Order

Sat, 16 May 2026 00:00:00 GMT

Find the k-th lexicographic number in [1, n] by counting subtree sizes in the digit trie.

K-th Smallest Prime Fraction

Sat, 16 May 2026 00:00:00 GMT

Among all i

Koko Eating Bananas

Sat, 16 May 2026 00:00:00 GMT

Binary search the minimum hourly eating speed Koko needs to finish all piles in h hours.

Kth Largest Element in a Stream

Sat, 16 May 2026 00:00:00 GMT

Maintain the kth-largest under a streaming add() using a fixed-size min-heap.

Kth Smallest Element in a BST

Sat, 16 May 2026 00:00:00 GMT

Iterative inorder traversal — pop the k-th node visited.

Kth Smallest Element in a Sorted Matrix

Sat, 16 May 2026 00:00:00 GMT

Row- and column-sorted matrix — binary search on value, or k-way merge over rows.

Kth Largest Element in an Array

Sat, 16 May 2026 00:00:00 GMT

Find the kth largest via min-heap of size k or quickselect for average O(n).

Kth Smallest Number in M Sorted Lists

Sat, 16 May 2026 00:00:00 GMT

Find the kth smallest value across m sorted arrays — pop k times from a min-heap of (value, listIdx, idxInList).

Kth Smallest Number in Multiplication Table

Sat, 16 May 2026 00:00:00 GMT

Binary search the value; count how many cells of the m-by-n multiplication table are at most mid via a per-row division.

Largest Number

Sat, 16 May 2026 00:00:00 GMT

Arrange integers to form the largest concatenated number — sort by custom comparator (a+b vs b+a).

Largest Number After Digit Swaps by Parity

Sat, 16 May 2026 00:00:00 GMT

Sort odd-positioned and even-positioned digits independently in descending order — two heaps.

Largest Rectangle in Histogram

Sat, 16 May 2026 00:00:00 GMT

Monotonic increasing stack — at every pop, the popped bar's rectangle spans from previous stack top to current index.

Largest Odd Number in String

Sat, 16 May 2026 00:00:00 GMT

Largest odd-valued substring of a digit string — scan from the right for the first odd digit.

Largest Palindromic Number

Sat, 16 May 2026 00:00:00 GMT

Build the largest palindrome from given digits using greedy half-construction.

Last Day Where You Can Still Cross

Sat, 16 May 2026 00:00:00 GMT

Latest day on which a path exists from top to bottom while cells flood — reverse Union-Find.

Lemonade Change

Sat, 16 May 2026 00:00:00 GMT

Greedy — always return the largest bill first when making change.

Letter Case Permutation

Sat, 16 May 2026 00:00:00 GMT

Generate every case-variant of a string — DFS with two choices per letter.

Least Number of Unique Integers after K Removals

Sat, 16 May 2026 00:00:00 GMT

Drop the rarest integers first — sort frequencies ascending and consume the budget.

Letter Tile Possibilities

Sat, 16 May 2026 00:00:00 GMT

Count distinct non-empty sequences buildable from tiles with possibly-duplicate letters — DFS over a frequency map.

Lexicographical Numbers

Sat, 16 May 2026 00:00:00 GMT

List 1..n in lexicographic (dictionary) order without sorting.

Letter Combinations of a Phone Number

Sat, 16 May 2026 00:00:00 GMT

Cartesian product of digit-to-letters mappings via DFS.

LFU Cache

Sat, 16 May 2026 00:00:00 GMT

O(1) LFU — per-frequency LRU bucket, key map, and an evolving min-freq counter.

Linked List Cycle II

Sat, 16 May 2026 00:00:00 GMT

Floyd's tortoise & hare — once they meet, restart one pointer from head; both step at speed 1 and converge at the cycle entry.

Linked List Cycle II

Sat, 16 May 2026 00:00:00 GMT

Find the node where a cycle begins using Floyd's two-phase detection.

Linked List Cycle

Sat, 16 May 2026 00:00:00 GMT

Detect a cycle in a singly linked list using Floyd's tortoise-and-hare.

Logger Rate Limiter

Sat, 16 May 2026 00:00:00 GMT

Allow a message at most once every 10 seconds — hash map of last-seen timestamps.

Linked List Cycle IV

Sat, 16 May 2026 00:00:00 GMT

curriculum-track follow-up — find the cycle entry and length; not a canonical LeetCode problem.

Longest Common Prefix

Sat, 16 May 2026 00:00:00 GMT

Find the longest common prefix among all strings — vertical scan or trie walk.

Longest Consecutive Sequence

Sat, 16 May 2026 00:00:00 GMT

Length of the longest run of consecutive integers in O(N) using a hash set.

Longest Common Subsequence

Sat, 16 May 2026 00:00:00 GMT

Length of the LCS of two strings — 2D DP rolling over rows.

Longest Common Suffix Queries

Sat, 16 May 2026 00:00:00 GMT

For each query string, find the word with longest common suffix — reversed-trie lookup.

Longest Cycle in a Graph

Sat, 16 May 2026 00:00:00 GMT

In a functional graph (out-degree 1), DFS each node with timestamps; revisiting a current-DFS node closes a cycle.

Longest Happy Prefix

Sat, 16 May 2026 00:00:00 GMT

Longest proper prefix of `s` that is also a suffix — KMP failure function.

Longest Happy String

Sat, 16 May 2026 00:00:00 GMT

Build the longest string with no three consecutive same characters — greedy max-heap by remaining count.

Longest Increasing Subsequence

Sat, 16 May 2026 00:00:00 GMT

Patience-sort tails — for each value, replace the leftmost tail ≥ it; final tail length is LIS.

Longest Increasing Path in a Matrix

Sat, 16 May 2026 00:00:00 GMT

Longest strictly-increasing path in any direction — DFS with memoization.

Longest Palindrome

Sat, 16 May 2026 00:00:00 GMT

Length of the longest palindrome buildable from the input letters — count pairs.

Longest Palindromic Substring

Sat, 16 May 2026 00:00:00 GMT

Expand around each center (n odd-center, n-1 even-center); track the longest match.

Longest Path With Different Adjacent Characters

Sat, 16 May 2026 00:00:00 GMT

Longest path in a tree such that no two adjacent nodes share a character — DFS combining top-two child paths per node.

Longest Palindrome by Concatenating Two-Letter Words

Sat, 16 May 2026 00:00:00 GMT

Max-length palindrome built from given 2-letter words — pair each word with its reverse.

Longest Subarray With Maximum Bitwise AND

Sat, 16 May 2026 00:00:00 GMT

The maximum AND equals the array's max — find the longest consecutive run of that value.

Longest Repeating Character Replacement

Sat, 16 May 2026 00:00:00 GMT

Longest substring achievable with at most k character replacements via a sliding window over character counts.

Longest Substring without Repeating Characters

Sat, 16 May 2026 00:00:00 GMT

Longest distinct-character substring via sliding window + last-seen index map.

Longest Valid Parentheses

Sat, 16 May 2026 00:00:00 GMT

Stack of indices anchored by the last invalid position; on each match, length = i - stack top.

Longest Subsequence With Limited Sum

Sat, 16 May 2026 00:00:00 GMT

Sort, prefix-sum, then upper_bound each query to find how many small items fit under the cap.

Lowest Common Ancestor of a Binary Search Tree

Sat, 16 May 2026 00:00:00 GMT

Walk down — go right if both values exceed node, left if both are less; otherwise current is the LCA.

Loud and Rich

Sat, 16 May 2026 00:00:00 GMT

DFS with memo — for each person, the answer is the minimum-quietness among self and all richer-than-self (transitively).

Longest Word With All Prefixes

Sat, 16 May 2026 00:00:00 GMT

Find the longest dictionary word whose every prefix is also in the dictionary — DFS on a trie of words.

Lowest Common Ancestor of a Binary Tree III

Sat, 16 May 2026 00:00:00 GMT

Find the LCA of two nodes when each node carries a parent pointer — the two-pointer "linked-list intersection" trick.

Lowest Common Ancestor of a Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Post-order DFS — return the first node whose left and right subtrees each contain one of p, q.

Lucky Numbers in a Matrix

Sat, 16 May 2026 00:00:00 GMT

Precompute per-row minimum and per-column maximum; an element is "lucky" iff it's both.

Magnetic Force Between Two Balls

Sat, 16 May 2026 00:00:00 GMT

Binary search the largest minimum gap such that m balls can be greedily placed in the sorted positions.

Map Sum Pairs

Sat, 16 May 2026 00:00:00 GMT

Insert (key, val) and query sum of values for keys with a given prefix — trie with subtree sums.

Majority Element

Sat, 16 May 2026 00:00:00 GMT

Boyer-Moore vote — increment count when seeing the candidate, decrement otherwise; reset candidate when count hits 0.

Matchsticks to Square

Sat, 16 May 2026 00:00:00 GMT

Can the matchsticks be partitioned into 4 equal-sum groups? Backtrack with sort-descending pruning.

Max Area of Island

Sat, 16 May 2026 00:00:00 GMT

DFS each unvisited land cell; flood-fill, count cells, track the maximum.

Max Consecutive Ones

Sat, 16 May 2026 00:00:00 GMT

Length of the longest run of 1s in a binary array.

Max Points on a Line

Sat, 16 May 2026 00:00:00 GMT

For each anchor, hash slopes in reduced (dx, dy) form — most popular slope gives the line through this anchor.

Max Stack

Sat, 16 May 2026 00:00:00 GMT

Stack with O(log n) popMax — ordered map of value to iterators into a linked list keeping insertion order.

Maximal Rectangle

Sat, 16 May 2026 00:00:00 GMT

Largest all-1 rectangle in a binary matrix — per-row "max rectangle in histogram" via monotonic stack.

Maximal Score After Applying K Operations

Sat, 16 May 2026 00:00:00 GMT

Pop the max k times, halving it each pop — max-heap with simulated operations.

Maximal Square

Sat, 16 May 2026 00:00:00 GMT

Largest all-1 square in a binary matrix — DP where dp[r][c] = 1 + min(left, top, top-left).

Maximum Area Rectangle With Point Constraints I

Sat, 16 May 2026 00:00:00 GMT

Find the largest axis-aligned rectangle whose four corners are in the point set and whose interior + boundary contain no other points.

Maximum Average Pass Ratio

Sat, 16 May 2026 00:00:00 GMT

Distribute extra students to maximize average class pass ratio — max-heap keyed by marginal improvement.

Maximum Average Subarray I

Sat, 16 May 2026 00:00:00 GMT

Maximum average of any length-k subarray — fixed-size sliding window over a running sum.

Maximum Depth of Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Depth is 1 + max(left depth, right depth); the empty tree has depth 0.

Maximum Number of Integers to Choose from a Range I

Sat, 16 May 2026 00:00:00 GMT

Greedily pick the smallest unbanned numbers from 1..n until you exceed maxSum.

Maximum Frequency Stack

Sat, 16 May 2026 00:00:00 GMT

Stack returning the most frequent element on pop, ties to the most recent — bucket stacks by frequency.

Maximum Number of Visible Points

Sat, 16 May 2026 00:00:00 GMT

Compute angles relative to viewer, sort, sliding-window an arc of size `angle` — handle wraparound by doubling the array.

Maximum Product After K Increments

Sat, 16 May 2026 00:00:00 GMT

With k +1 operations, maximize the product — always increment the current smallest via a min-heap.

Maximum Performance of a Team

Sat, 16 May 2026 00:00:00 GMT

Performance = sum(speed) * min(efficiency); sort by efficiency desc, keep top-k speeds.

Maximum Product Subarray

Sat, 16 May 2026 00:00:00 GMT

Max product of any contiguous subarray — track current max AND current min (negatives flip).

Maximum Profit in Job Scheduling

Sat, 16 May 2026 00:00:00 GMT

Sort jobs by end; dp[i] = max(skip dp[i-1], take profit + dp[predecessor end ≤ start]); binary-search the predecessor.

Maximum Running Time of N Computers

Sat, 16 May 2026 00:00:00 GMT

With k batteries (any one can power any computer, swappable instantly), maximize minutes all n computers can run — binary search on answer.

Maximum Subarray

Sat, 16 May 2026 00:00:00 GMT

Kadane's — running sum reset to current value whenever it goes negative; track running max.

Maximum Swap

Sat, 16 May 2026 00:00:00 GMT

Swap at most two digits once to make a number as large as possible — last-occurrence lookup.

Maximum Value at a Given Index in a Bounded Array

Sat, 16 May 2026 00:00:00 GMT

Build a length-n positive-integer array with adjacent diff ≤ 1, total sum ≤ maxSum — binary search the peak.

Maximum Twin Sum of a Linked List

Sat, 16 May 2026 00:00:00 GMT

Pair the i-th node with the (n-1-i)-th node; return the max twin sum using the middle-reverse trick.

Median of Two Sorted Arrays

Sat, 16 May 2026 00:00:00 GMT

Binary search on the partition of the smaller array — find a split with equal-sized halves and left ≤ right on both sides.

Maximum Width Ramp

Sat, 16 May 2026 00:00:00 GMT

Build a decreasing-index stack on left pass; right-to-left, pop while top is less-equal current and record gap.

Meeting Rooms II

Sat, 16 May 2026 00:00:00 GMT

Minimum rooms required for all meetings — sweep starts and ends with a heap (or sorted-arrays merge).

Meeting Rooms

Sat, 16 May 2026 00:00:00 GMT

Sort by start — if any meeting begins before the previous one ends, the schedule conflicts.

Meeting Rooms III

Sat, 16 May 2026 00:00:00 GMT

With fixed n rooms, schedule meetings preferring the lowest-numbered free room — return the busiest room.

Merge Intervals

Sat, 16 May 2026 00:00:00 GMT

Merge overlapping intervals — sort by start, then sweep linearly.

Merge K Sorted Lists

Sat, 16 May 2026 00:00:00 GMT

Merge k sorted linked lists using a min-heap of current heads.

Merge Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Merge nums2 into nums1 in place, filling from the back to avoid overwrites.

Merge Two Sorted Lists

Sat, 16 May 2026 00:00:00 GMT

Dummy-head merge — splice the smaller head into the tail at each step.

Middle of the Linked List

Sat, 16 May 2026 00:00:00 GMT

Find the middle node in one pass using the speed-1 / speed-2 pointer pair.

Min Cost Climbing Stairs

Sat, 16 May 2026 00:00:00 GMT

Min cost to reach the top (past the last step) using 1- or 2-step moves from positions 0 or 1.

Minimize Manhattan Distances

Sat, 16 May 2026 00:00:00 GMT

Convert (x,y) to rotated (x+y, x-y) so Manhattan becomes Chebyshev — track top-2 extremes to handle "remove one point".

Minimize Malware Spread

Sat, 16 May 2026 00:00:00 GMT

Remove one initially infected node to minimize total infections — DSU on the network.

Min Stack

Sat, 16 May 2026 00:00:00 GMT

Stack with O(1) getMin — each entry carries the min of itself and everything below.

Minimize Maximum Value in a Grid

Sat, 16 May 2026 00:00:00 GMT

Sort all cells by value, then assign each its rank under "max(row max so far, col max so far) + 1".

Minimize Max Distance to Gas Station

Sat, 16 May 2026 00:00:00 GMT

Add k gas stations on a number line to minimize the maximum gap — binary search on the answer with a greedy feasibility check.

Minimum Cost to Hire K Workers

Sat, 16 May 2026 00:00:00 GMT

Sort by ratio, then maintain a max-heap of qualities — pay the minimum possible at each ratio threshold.

Minimum Area Rectangle

Sat, 16 May 2026 00:00:00 GMT

Axis-aligned rectangles — group points by x, for every pair of x-columns find shared y-pairs and compute area.

Minimum Cost to Connect Sticks

Sat, 16 May 2026 00:00:00 GMT

Huffman-style merge — always combine the two shortest sticks via a min-heap.

Minimum Cost to Make at Least One Valid Path in a Grid

Sat, 16 May 2026 00:00:00 GMT

0-1 BFS — following the arrow costs 0, changing direction costs 1.

Minimum Cuts to Divide a Circle

Sat, 16 May 2026 00:00:00 GMT

Closed-form — 0 for n=1, 1 for odd n, n/2 for even n (diameter cuts).

Minimum Flips to Make the Binary String Alternate

Sat, 16 May 2026 00:00:00 GMT

Doubled string sliding window — for each length-n window, count mismatches against "0101..." and "1010..." patterns.

Minimum Height Trees

Sat, 16 May 2026 00:00:00 GMT

BFS from all leaves inward, peeling layers; the last 1 or 2 nodes form the centroid set.

Minimum Interval to Include Each Query

Sat, 16 May 2026 00:00:00 GMT

For each query, return the length of the shortest interval containing it — offline sweep with a min-heap.

Minimum Moves to Spread Stones Over Grid

Sat, 16 May 2026 00:00:00 GMT

3×3 grid with 9 stones across cells (some empty, some doubled) — backtrack to match donors and receivers by min Manhattan distance.

Minimum Number of K Consecutive Bit Flips

Sat, 16 May 2026 00:00:00 GMT

Greedy left-to-right — flip a length-k window whenever the current bit is 0, track active flips via a deferred-update marker.

Minimum Number of Lines to Cover Points

Sat, 16 May 2026 00:00:00 GMT

Bitmask DP — for each subset, try every line through two uncovered points; minimize lines.

Minimum Number of Moves to Make Palindrome

Sat, 16 May 2026 00:00:00 GMT

Greedy two-pointer count of adjacent swaps needed to make a string a palindrome.

Minimum Number of Refueling Stops

Sat, 16 May 2026 00:00:00 GMT

Minimum stops to reach the target — when you "run out", retroactively take the largest skipped tank from a max-heap.

Minimum Number of Pushes to Type Word II

Sat, 16 May 2026 00:00:00 GMT

Map letters to phone keys greedily — most frequent letters get position 1.

Minimum One Bit Operations to Make Integers Zero

Sat, 16 May 2026 00:00:00 GMT

Gray-code position equals the answer — XOR-fold all bits with cumulative XOR scan from the top.

Minimum Operations to Make All Array Elements Equal

Sat, 16 May 2026 00:00:00 GMT

For each query, sort once and use prefix sums plus binary search to compute the total deviation.

Minimum Remove to Make Valid Parentheses

Sat, 16 May 2026 00:00:00 GMT

Stack indices of unmatched openers; on close-without-opener, mark the closer for removal — then strip.

Minimum Path Sum

Sat, 16 May 2026 00:00:00 GMT

Minimum sum from top-left to bottom-right moving right/down — 1D rolling DP.

Minimum Replacements to Sort the Array

Sat, 16 May 2026 00:00:00 GMT

Replace each oversized element with k equal parts ≤ next; walk right-to-left tracking the running ceiling.

Minimum Space Wasted from Packaging

Sat, 16 May 2026 00:00:00 GMT

For each supplier, sort boxes, then per-package binary search for the smallest fitting box and accumulate waste.

Minimum String Length After Removing Substrings

Sat, 16 May 2026 00:00:00 GMT

Push characters; when the top with the current forms "AB" or "CD", pop instead — return final length.

Minimum Size Subarray Sum

Sat, 16 May 2026 00:00:00 GMT

Shortest contiguous subarray with sum ≥ target via expanding/contracting window over positive integers.

Minimum Time Takes to Reach Destination Without Drowning

Sat, 16 May 2026 00:00:00 GMT

BFS with a time-dependent flood wavefront preventing entry into recently flooded cells.

Minimum Time Visiting All Points

Sat, 16 May 2026 00:00:00 GMT

Each pair-to-pair time is Chebyshev distance — max of |dx|, |dy|. Sum across the route.

Minimum Window Substring

Sat, 16 May 2026 00:00:00 GMT

Smallest substring of s covering every character of t — variable-size sliding window with a 'missing' counter.

Minimum Window Subsequence

Sat, 16 May 2026 00:00:00 GMT

Smallest substring of S containing T as a subsequence (not contiguously) — two-pointer back-and-forth scan.

Missing Number

Sat, 16 May 2026 00:00:00 GMT

Find the missing number in [0, n] — XOR sum trick or arithmetic sum.

Moving Average from Data Stream

Sat, 16 May 2026 00:00:00 GMT

Sliding window average — fixed-size deque holding the last k values, maintain a running sum.

Multiply Strings

Sat, 16 May 2026 00:00:00 GMT

Schoolbook product into an int array — position i*j contributes to slots i+j and i+j+1; final carry-resolution sweep.

Most Stones Removed with Same Row or Column

Sat, 16 May 2026 00:00:00 GMT

Max stones removable while keeping each connected component non-empty — DSU on row/column unions.

N-Queens II

Sat, 16 May 2026 00:00:00 GMT

Count the n-queens solutions using bitmask backtracking over columns, diagonals, and anti-diagonals.

N-Queens

Sat, 16 May 2026 00:00:00 GMT

Return all distinct n-queens placements as board strings — bitmask backtracking with board reconstruction.

N-th Tribonacci Number

Sat, 16 May 2026 00:00:00 GMT

T(n) = T(n-1) + T(n-2) + T(n-3); rolling 3 scalars.

N-Repeated Element in Size 2N Array

Sat, 16 May 2026 00:00:00 GMT

Find the value that appears N times in a 2N array — hash set or adjacent-pair scan.

Network Delay Time

Sat, 16 May 2026 00:00:00 GMT

Dijkstra from k; the answer is the maximum shortest-path distance (or -1 if any node is unreachable).

Nested List Weight Sum II

Sat, 16 May 2026 00:00:00 GMT

Find max depth, then weight each integer by (maxDepth - itsDepth + 1).

Next Greater Element IV

Sat, 16 May 2026 00:00:00 GMT

Two monotonic stacks — first finds the next greater; the second collects those popped to find the second.

Next Palindrome Using Same Digits

Sat, 16 May 2026 00:00:00 GMT

Find the next-greater palindrome using the same digit multiset as the input.

Next Greater Element I

Sat, 16 May 2026 00:00:00 GMT

For each element in nums1, find its next greater element in nums2 using a monotonic stack.

Next Permutation

Sat, 16 May 2026 00:00:00 GMT

Rearrange to the lexicographically next greater permutation; wrap to lowest if impossible.

Nim Game

Sat, 16 May 2026 00:00:00 GMT

You lose iff n is a multiple of 4 — both players play optimally with 1, 2, or 3 stones per turn.

Non-overlapping Intervals

Sat, 16 May 2026 00:00:00 GMT

Greedy by earliest end — remove any interval that starts before the last kept end.

Number of 1 Bits

Sat, 16 May 2026 00:00:00 GMT

Brian Kernighan's loop — `n &= n - 1` clears the lowest set bit each iteration.

Nth Magical Number

Sat, 16 May 2026 00:00:00 GMT

Binary search on the answer — count magical numbers ≤ x as `x/a + x/b - x/lcm(a,b)`.

Number of Connected Components in an Undirected Graph

Sat, 16 May 2026 00:00:00 GMT

Union-find each edge; count distinct roots.

Number of Distinct Islands

Sat, 16 May 2026 00:00:00 GMT

Count distinct island shapes by canonicalizing each via DFS path signature.

Number of Flowers in Full Bloom

Sat, 16 May 2026 00:00:00 GMT

For each query time, count blooming flowers using two sorted endpoint arrays and binary search.

Number of Islands II

Sat, 16 May 2026 00:00:00 GMT

Stream of land additions on a grid — report island count after each.

Number of Provinces

Sat, 16 May 2026 00:00:00 GMT

Union-find over the friendship matrix — count distinct roots.

Number of Islands

Sat, 16 May 2026 00:00:00 GMT

Count 4-connected components of 1s in a binary grid — DFS or Union-Find.

Number of Longest Increasing Subsequence

Sat, 16 May 2026 00:00:00 GMT

Count distinct longest increasing subsequences — DP carrying (length, count) per index.

Number of Steps to Reduce a Binary Number to One

Sat, 16 May 2026 00:00:00 GMT

Process the binary string from LSB; each set bit triggers an add (carry chain) that may cascade — simulate with a carry flag.

Number of Substrings Containing All Three Characters

Sat, 16 May 2026 00:00:00 GMT

Count substrings containing at least one a, b, and c — sliding window with `ans += left` trick.

Number of Spaces Cleaning Robot Cleaned

Sat, 16 May 2026 00:00:00 GMT

Simulate a robot that turns right on obstacles, halting once it revisits a state already visited in the same direction.

Number of Visible People in a Queue

Sat, 16 May 2026 00:00:00 GMT

Right-to-left monotonic decreasing stack — each pop counts a visible person.

Number of Valid Subarrays

Sat, 16 May 2026 00:00:00 GMT

For each index, count subarrays ending at it where nums[i] is the leftmost minimum — monotonic stack.

Number of Ways to Form a Target String Given a Dictionary

Sat, 16 May 2026 00:00:00 GMT

Count ways to form target by picking one char from each column-position of `words` — 2D DP.

Number of Wonderful Substrings

Sat, 16 May 2026 00:00:00 GMT

Count substrings where at most one letter has an odd count — prefix bitmask + hash map.

Odd Even Linked List

Sat, 16 May 2026 00:00:00 GMT

Reorder a list so odd-indexed nodes come first, then even-indexed, preserving relative order — two interleaved chains.

Open the Lock

Sat, 16 May 2026 00:00:00 GMT

Shortest dial sequence to reach a 4-digit target avoiding deadends — BFS over 10000 states.

Pacific Atlantic Water Flow

Sat, 16 May 2026 00:00:00 GMT

BFS/DFS from each ocean's edge cells upward — intersection of reachable sets is the answer.

Optimal Account Balancing

Sat, 16 May 2026 00:00:00 GMT

Minimum transactions to clear all debts — DFS over net balances trying offsetting pairs.

Pairs of Songs With Total Durations Divisible by 60

Sat, 16 May 2026 00:00:00 GMT

Count pairs `(i, j)` with `(t[i] + t[j]) % 60 == 0` — residue bucketing.

Optimize Water Distribution in a Village

Sat, 16 May 2026 00:00:00 GMT

Min cost to supply water to all houses — MST with a virtual well source.

Palindrome Number

Sat, 16 May 2026 00:00:00 GMT

Reverse the lower half — equals (or with-middle-digit equals) the upper half.

Palindrome Linked List

Sat, 16 May 2026 00:00:00 GMT

Check linked-list palindromicity by reversing the second half in place and walking both halves.

Palindrome Pairs

Sat, 16 May 2026 00:00:00 GMT

Find all index pairs (i, j) such that words[i] + words[j] is a palindrome — hash-map prefix/suffix trick.

Palindrome Permutation

Sat, 16 May 2026 00:00:00 GMT

Check whether any permutation of `s` is a palindrome — count odd-frequency letters.

Palindromic Substrings

Sat, 16 May 2026 00:00:00 GMT

Count palindromic substrings of s using expand-around-center in O(n²).

Parallel Courses III

Sat, 16 May 2026 00:00:00 GMT

Minimum time to finish all courses given per-course time and prerequisites — topological DP.

Parallel Courses

Sat, 16 May 2026 00:00:00 GMT

Minimum number of semesters to complete n courses with dependencies, unlimited parallelism — BFS over semester levels.

Parsing A Boolean Expression

Sat, 16 May 2026 00:00:00 GMT

Stack characters until `)`; pop until `(`, evaluate the operator above, and push the result.

Partition Equal Subset Sum (Bitset Variant)

Sat, 16 May 2026 00:00:00 GMT

The same problem as Batch 1 — this duplicate row in the curriculum gets a bitset-accelerated take.

0/1 Knapsack — Partition Equal Subset Sum

Sat, 16 May 2026 00:00:00 GMT

Can we split nums into two equal-sum subsets? Classical 0/1 knapsack DP.

Partition Labels

Sat, 16 May 2026 00:00:00 GMT

Cut a string into the maximum number of parts such that no letter appears in more than one part.

Pascal's Triangle

Sat, 16 May 2026 00:00:00 GMT

Build the first numRows of Pascal's Triangle — row[j] = prevRow[j-1] + prevRow[j].

Path with Maximum Probability

Sat, 16 May 2026 00:00:00 GMT

Dijkstra with max-heap on the product of edge probabilities.

Path Sum

Sat, 16 May 2026 00:00:00 GMT

DFS subtracting each node's value; at a leaf, return whether the remainder is exactly zero.

Paths in Maze That Lead to Same Room

Sat, 16 May 2026 00:00:00 GMT

Count triangles in an undirected graph — for each edge (u, v), count neighbors shared by both, then divide.

Perfect Squares

Sat, 16 May 2026 00:00:00 GMT

BFS by levels (or DP) — at each level subtract a perfect square, return the first level reaching 0.

Permutation in String

Sat, 16 May 2026 00:00:00 GMT

Does s2 contain any permutation of s1? Fixed-size sliding window over a frequency vector.

Permutations II

Sat, 16 May 2026 00:00:00 GMT

Backtracking with a "use-leftmost-unused-duplicate" tie-breaker — sort, then skip a value if its previous copy is unused.

Plus One

Sat, 16 May 2026 00:00:00 GMT

Increment the LSB; on 9 set to 0 and carry; if every digit was 9, prepend 1.

Permutations

Sat, 16 May 2026 00:00:00 GMT

Generate all permutations of a unique-element array via DFS with in-place swap-based backtracking.

Poor Pigs

Sat, 16 May 2026 00:00:00 GMT

Each pig encodes a base-(rounds+1) digit — smallest p with (rounds+1)^p >= buckets is the answer.

Power of Three

Sat, 16 May 2026 00:00:00 GMT

The largest int32 power of 3 is `3^19 = 1162261467` — check whether n divides it.

Populating Next Right Pointers in Each Node

Sat, 16 May 2026 00:00:00 GMT

Wire each node's `next` pointer to its right neighbor in a perfect binary tree using O(1) extra space.

Power of Two

Sat, 16 May 2026 00:00:00 GMT

A positive power of two has exactly one set bit — `n & (n - 1) == 0`.

Powerful Integers

Sat, 16 May 2026 00:00:00 GMT

All values `x^i + y^j <= bound` for non-negative i, j — bounded brute force with a set.

Product of Array Except Self

Sat, 16 May 2026 00:00:00 GMT

Two passes — left prefix products into output, then right prefix on the fly multiplying in.

Put Marbles in Bags

Sat, 16 May 2026 00:00:00 GMT

Each cut between consecutive marbles contributes a pair-sum; the answer is the spread between the largest k-1 and smallest k-1 pair-sums.

Queries on Number of Points Inside a Circle

Sat, 16 May 2026 00:00:00 GMT

For each query, count points within radius — brute force with squared distance is fast enough.

Range Sum Query - Immutable

Sat, 16 May 2026 00:00:00 GMT

Prefix-sum precomputation — O(1) range sum after O(n) build.

Range Sum of Sorted Subarray Sums

Sat, 16 May 2026 00:00:00 GMT

Generate all subarray sums, sort, and sum the requested index range modulo 1e9+7.

Range Module

Sat, 16 May 2026 00:00:00 GMT

Track tracked half-open intervals — std::map keyed by start, with merge/split on add/remove and predecessor lookup for queries.

Rearranging Fruits

Sat, 16 May 2026 00:00:00 GMT

Minimum cost to equalize two baskets by swapping — sort the symmetric-difference half and use the global min as a "courier".

Rank Teams by Votes

Sat, 16 May 2026 00:00:00 GMT

Rank teams by per-position vote tallies, ties broken by team name.

Ransom Note

Sat, 16 May 2026 00:00:00 GMT

Can `ransomNote` be built from `magazine`'s letters? Letter-count subset check.

Reconstruct Itinerary

Sat, 16 May 2026 00:00:00 GMT

Hierholzer's algorithm — pop edges greedily in lexicographic order and append to the route on backtrack.

Recover a Tree From Preorder Traversal

Sat, 16 May 2026 00:00:00 GMT

Parse depth-prefix tokens; maintain a stack of ancestors and attach each node to its parent at depth-1.

Rectangle Area

Sat, 16 May 2026 00:00:00 GMT

Total = sum of areas minus the overlap area; overlap dims are max(0, min - max) on each axis.

Redundant Connection

Sat, 16 May 2026 00:00:00 GMT

Find the edge that creates a cycle in a tree-augmented graph — Union-Find detects the merge that closes a loop.

Rectangle Overlap

Sat, 16 May 2026 00:00:00 GMT

Two axis-aligned rectangles overlap iff their projections on both axes overlap (strictly, not just touch).

Regions Cut by Slashes

Sat, 16 May 2026 00:00:00 GMT

Count regions cut by slashes in an `n x n` grid by subdividing each cell into 4 triangles.

Remove All Adjacent Duplicates In String

Sat, 16 May 2026 00:00:00 GMT

Use a stack (or string accumulator) — push or pop based on the top.

Regular Expression Matching

Sat, 16 May 2026 00:00:00 GMT

Match s against p with `.` and `*` — 2D DP over prefix matching.

Remove Duplicate Letters

Sat, 16 May 2026 00:00:00 GMT

Greedy monotonic stack — keep the lexicographically smallest string with each letter exactly once.

Remove Covered Intervals

Sat, 16 May 2026 00:00:00 GMT

Count intervals not covered by another after sorting by start ascending, end descending.

Remove Duplicates from Sorted List

Sat, 16 May 2026 00:00:00 GMT

Drop adjacent duplicates from a sorted singly linked list with one pass.

Remove Duplicates from Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Compress consecutive duplicates in a sorted array in place — single write pointer.

Remove Element

Sat, 16 May 2026 00:00:00 GMT

Remove all occurrences of val from an array in place; return the new length.

Remove K Digits

Sat, 16 May 2026 00:00:00 GMT

Remove k digits from a number-string to make the smallest result — monotonic increasing stack.

Remove Invalid Parentheses

Sat, 16 May 2026 00:00:00 GMT

Remove the minimum parentheses to make s valid; return all distinct resulting strings via BFS or targeted DFS.

Remove Linked List Elements

Sat, 16 May 2026 00:00:00 GMT

Delete every node with the given value — dummy head + walking predecessor.

Reorder List

Sat, 16 May 2026 00:00:00 GMT

Reorder `L0 → L1 → … → Ln` into `L0 → Ln → L1 → Ln-1 → …` by splitting, reversing, and weaving in place.

Remove nth Node from End of List

Sat, 16 May 2026 00:00:00 GMT

Drop the nth-from-last node in one pass using a fast/slow pointer gap of n.

Reorder Routes to Make All Paths Lead to the City Zero

Sat, 16 May 2026 00:00:00 GMT

BFS/DFS the undirected tree from city 0; count edges that point away — those must be flipped.

Reorganize String

Sat, 16 May 2026 00:00:00 GMT

Rearrange so no two adjacent chars are equal — greedy max-heap by remaining count.

Repeated DNA Sequences

Sat, 16 May 2026 00:00:00 GMT

Find every 10-letter DNA substring that appears more than once via fixed-size sliding window + hash set.

Replace Words

Sat, 16 May 2026 00:00:00 GMT

Replace every word in a sentence with the shortest dictionary root that prefixes it.

Restore IP Addresses

Sat, 16 May 2026 00:00:00 GMT

Enumerate all valid IPv4 addresses formed by inserting three dots into a digit string — DFS with strict octet rules.

Reverse Bits

Sat, 16 May 2026 00:00:00 GMT

Reverse 32 bits of an unsigned int — shift the result left and OR with the low bit of the input each step.

Reverse Linked List

Sat, 16 May 2026 00:00:00 GMT

In-place iterative reversal using prev/curr/next bookkeeping — the foundational linked-list primitive.

Reverse Integer

Sat, 16 May 2026 00:00:00 GMT

Reverse a 32-bit signed integer's digits — detect overflow before multiplying by 10.

Reverse Linked List II

Sat, 16 May 2026 00:00:00 GMT

Reverse the sublist between positions left and right in place using the dummy-head + head-insertion trick.

Reverse Nodes In Even Length Groups

Sat, 16 May 2026 00:00:00 GMT

Reverse each natural-numbered group of nodes whose length is even.

Reverse String

Sat, 16 May 2026 00:00:00 GMT

In-place reversal of a character array using converging two pointers.

Reverse Nodes in k-Group

Sat, 16 May 2026 00:00:00 GMT

Reverse every consecutive group of k nodes in place; leave the trailing remainder untouched.

Reverse Pairs

Sat, 16 May 2026 00:00:00 GMT

Count pairs where i precedes j and nums[i] exceeds twice nums[j] using a merge-sort with cross-count.

Reverse Vowels of a String

Sat, 16 May 2026 00:00:00 GMT

Swap only the vowel characters using converging pointers that skip non-vowels.

Reverse Words in a String

Sat, 16 May 2026 00:00:00 GMT

Reverse the order of whitespace-delimited words, collapsing extra spaces.

Roman to Integer

Sat, 16 May 2026 00:00:00 GMT

Parse a Roman numeral into its integer value — subtract when a smaller numeral precedes a larger.

Rotate Array

Sat, 16 May 2026 00:00:00 GMT

Rotate an array right by k steps in place using three reverses.

Rotate Image

Sat, 16 May 2026 00:00:00 GMT

Transpose then reverse each row — a 90-degree clockwise rotation in O(1) extra space.

Rotate List

Sat, 16 May 2026 00:00:00 GMT

Rotate a singly linked list right by k positions by closing the cycle, advancing, and cutting.

Rotting Oranges

Sat, 16 May 2026 00:00:00 GMT

Multi-source BFS — seed the queue with all initially-rotten oranges and level-iterate; return the level when the queue empties.

Russian Doll Envelopes

Sat, 16 May 2026 00:00:00 GMT

Sort by width ascending, height descending; then longest-increasing-subsequence on heights.

Schedule Tasks on Minimum Machines

Sat, 16 May 2026 00:00:00 GMT

curriculum variant of meeting rooms — schedule interval-style tasks across the fewest possible machines.

Search in Rotated Sorted Array II

Sat, 16 May 2026 00:00:00 GMT

Same as version I but values may duplicate — strip equal endpoints to recover monotonicity, worst-case O(n).

Search in Rotated Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Binary search a rotated sorted array — one half is always sorted; route the search there.

Same Tree

Sat, 16 May 2026 00:00:00 GMT

Structural recursion — both null OK; one null fails; equal values + recurse left and right.

Search Suggestions System

Sat, 16 May 2026 00:00:00 GMT

For each prefix of searchWord, return up to 3 lexicographically smallest matching products.

Self Crossing

Sat, 16 May 2026 00:00:00 GMT

Three geometric cases — the path crosses iff one of "4th edge crosses 1st", "5th meets 1st", or "6th crosses 1st" holds.

Serialize and Deserialize Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Preorder serialization with null markers; deserialize by recursively consuming the next token.

Search Insert Position

Sat, 16 May 2026 00:00:00 GMT

Find the index of target in a sorted array, or the index where it would be inserted to maintain order.

Set Matrix Zeros

Sat, 16 May 2026 00:00:00 GMT

Use the matrix's own first row and column as zero-flag storage to achieve O(1) extra space.

Set Mismatch

Sat, 16 May 2026 00:00:00 GMT

Find the duplicated and missing values in [1, n] via cyclic sort.

Shortest Bridge

Sat, 16 May 2026 00:00:00 GMT

Find one island via DFS, then multi-source BFS outward from it; first time we hit the second island is the answer.

Shortest Common Supersequence

Sat, 16 May 2026 00:00:00 GMT

Shortest string containing both s1 and s2 as subsequences — LCS DP then reconstruction.

Shortest Cycle in a Graph

Sat, 16 May 2026 00:00:00 GMT

For each source node, BFS to find the shortest cycle including it — return the global minimum.

Shortest Path Visiting All Nodes

Sat, 16 May 2026 00:00:00 GMT

BFS over (node, visited-bitmask) states — return path length when the bitmask is all-ones.

Shortest Word Distance II

Sat, 16 May 2026 00:00:00 GMT

Repeated distance queries over a fixed word list — preprocess word to sorted index list, two-pointer merge per query.

Similar String Groups

Sat, 16 May 2026 00:00:00 GMT

Group strings where two differ by exactly one swap — DSU on pairwise similarity.

Single Element in a Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Sorted array where every value appears twice except one — binary search on pair-aligned indices.

Single Number II

Sat, 16 May 2026 00:00:00 GMT

Every value appears three times except one — count bits mod 3 using two state bits (ones, twos).

Single Number

Sat, 16 May 2026 00:00:00 GMT

XOR-fold the whole array — pairs cancel, the lone unmatched value remains.

Smallest Range Covering Elements from K Lists

Sat, 16 May 2026 00:00:00 GMT

Smallest [lo, hi] covering at least one element from each of k sorted lists — k-way merge tracking running max.

Sliding Window Maximum

Sat, 16 May 2026 00:00:00 GMT

Max of every length-k window via a monotonic deque.

Snapshot Array

Sat, 16 May 2026 00:00:00 GMT

Versioned array with O(1) snap and O(log s) lookup — store change history per index, binary search by snap id.

Sliding Window Median

Sat, 16 May 2026 00:00:00 GMT

Median of every length-k window — two multisets with lazy rebalance.

Sort an Array

Sat, 16 May 2026 00:00:00 GMT

Sort an array in O(n log n) using merge sort or randomized quicksort.

Sort Array By Parity II

Sat, 16 May 2026 00:00:00 GMT

Even-valued at even indices, odd-valued at odd indices — two interleaved pointers.

Smallest Rectangle Enclosing Black Pixels

Sat, 16 May 2026 00:00:00 GMT

Binary search the four bounding rows/columns by checking if each side contains any black pixel.

Sort Items by Groups Respecting Dependencies

Sat, 16 May 2026 00:00:00 GMT

Order items so that group membership stays contiguous and dependencies are respected — nested topological sorts.

Sort Colors

Sat, 16 May 2026 00:00:00 GMT

In-place sort of an array containing only 0, 1, and 2 using the Dutch-national-flag three-pointer partition.

Sort List

Sat, 16 May 2026 00:00:00 GMT

Top-down merge sort on a linked list — slow/fast split, recurse, merge two sorted lists.

Spiral Matrix II

Sat, 16 May 2026 00:00:00 GMT

Generate an n-by-n matrix filled 1..n^2 by walking the same four-direction spiral as Spiral Matrix.

Spiral Matrix

Sat, 16 May 2026 00:00:00 GMT

Peel layers in four-direction order, shrinking the active rectangle after each side.

Split a Circular Linked List

Sat, 16 May 2026 00:00:00 GMT

curriculum variant — split a circular linked list into two roughly equal halves, each itself circular.

Split a String Into the Max Number of Unique Substrings

Sat, 16 May 2026 00:00:00 GMT

DFS partition of the string trying every prefix; track a set of already-used substrings.

Split Array Into Two Arrays to Minimize Sum Difference

Sat, 16 May 2026 00:00:00 GMT

Partition 2n integers into two n-sized halves minimizing |sumA − sumB| — meet-in-the-middle + binary search.

Split Array Largest Sum

Sat, 16 May 2026 00:00:00 GMT

Partition the array into m contiguous parts minimizing the maximum part sum — binary search on the answer.

Squares of a Sorted Array

Sat, 16 May 2026 00:00:00 GMT

Given a sorted (possibly negative) array, return the squares sorted ascending — two-pointer merge from both ends.

Split Linked List in Parts

Sat, 16 May 2026 00:00:00 GMT

Split a list into k roughly-equal parts (earlier parts at most one node larger).

String to Integer (atoi)

Sat, 16 May 2026 00:00:00 GMT

Skip whitespace, parse optional sign, consume digits with overflow clamp.

Stream of Characters

Sat, 16 May 2026 00:00:00 GMT

Streaming multi-pattern match — build an Aho-Corasick-like trie over reversed words, walk it with a buffer of recent chars.

String Compression

Sat, 16 May 2026 00:00:00 GMT

Compress runs of identical characters in place, writing the character followed by its count.

Strobogrammatic Number

Sat, 16 May 2026 00:00:00 GMT

Check whether a numeric string looks the same when rotated 180°.

Subarrays with K Different Integers

Sat, 16 May 2026 00:00:00 GMT

Count subarrays with exactly K distinct integers — atMost(K) − atMost(K−1).

Subsets II

Sat, 16 May 2026 00:00:00 GMT

Enumerate all distinct subsets when the input may contain duplicates — sort and skip duplicate branches.

Strong Password Checker

Sat, 16 May 2026 00:00:00 GMT

Compute the minimum edits (insert, delete, replace) to make a password "strong" — careful case analysis around triplet runs.

Subsets

Sat, 16 May 2026 00:00:00 GMT

Enumerate all subsets of a unique array — bitmask, recursion, or iterative "double" pattern.

Subtree of Another Tree

Sat, 16 May 2026 00:00:00 GMT

DFS over root; at each node, run an exact-match check against subRoot.

Substring with Concatenation of All Words

Sat, 16 May 2026 00:00:00 GMT

Find all start indices where s contains a concatenation of every word in `words` in any order.

Sum of All Subset XOR Totals

Sat, 16 May 2026 00:00:00 GMT

Each bit set in any element appears in exactly half the subsets — answer is OR-of-all-bits times 2^(n-1).

Sudoku Solver

Sat, 16 May 2026 00:00:00 GMT

Solve a Sudoku puzzle in place via backtracking with row/col/box bitmasks.

Sum of Mutated Array Closest to Target

Sat, 16 May 2026 00:00:00 GMT

Binary search the cap value that, after clamping every element, makes the array sum closest to target.

Sum of Distances in a Tree

Sat, 16 May 2026 00:00:00 GMT

Rerooting — first DFS computes subtree sums; second DFS shifts the perspective using the edge swing formula.

Sum of Two Integers

Sat, 16 May 2026 00:00:00 GMT

Add without `+` — XOR is sum without carry; AND-shifted is the carry; loop until carry is zero.

Super Ugly Number

Sat, 16 May 2026 00:00:00 GMT

The nth number whose prime factors lie in a given list — k-way merge of multiples.

Swap Nodes in Pairs

Sat, 16 May 2026 00:00:00 GMT

Swap every two adjacent nodes in place using a dummy and a sliding triple.

Swim in Rising Water

Sat, 16 May 2026 00:00:00 GMT

Dijkstra where edge weight is the destination cell's height — the path cost is the max along the path.

Symmetric Tree

Sat, 16 May 2026 00:00:00 GMT

Check whether a binary tree is a mirror of itself by comparing left/right subtrees pairwise.

Swapping Nodes in a Linked List

Sat, 16 May 2026 00:00:00 GMT

Swap values (not nodes) of the kth-from-start and kth-from-end positions with a single offset-pointer pass.

Task Scheduler

Sat, 16 May 2026 00:00:00 GMT

Schedule tasks with a cooldown using a math identity on the most-frequent task.

Text Justification

Sat, 16 May 2026 00:00:00 GMT

Pack words greedily per line; distribute spaces evenly with extras on the left, left-justify the last line.

The K Weakest Rows in a Matrix

Sat, 16 May 2026 00:00:00 GMT

Rank rows by (soldiers, index) and return the k weakest — binary search soldier counts plus a heap.

The Number of Good Subsets

Sat, 16 May 2026 00:00:00 GMT

Count subsets whose product is a product of distinct primes — bitmask DP over the 10 primes ≤ 30.

The Number of the Smallest Unoccupied Chair

Sat, 16 May 2026 00:00:00 GMT

Friends arrive and leave at intervals — return which chair the target friend sits on, using two heaps.

The Skyline Problem

Sat, 16 May 2026 00:00:00 GMT

Compute the contour of building rectangles using a max-heap sweep over critical x-coordinates.

Third Maximum Number

Sat, 16 May 2026 00:00:00 GMT

Return the third distinct maximum (or the maximum if fewer than 3 distinct exist) — three running tracker variables.

Top K Frequent Elements

Sat, 16 May 2026 00:00:00 GMT

Return the k most frequent elements via bucket sort by frequency.

Time-Based Key-Value Store

Sat, 16 May 2026 00:00:00 GMT

set/get keyed by timestamp — per-key sorted timestamps with binary search for the largest timestamp not exceeding the query.

Total Appeal of a String

Sat, 16 May 2026 00:00:00 GMT

Sum, over every substring, the count of distinct characters — contribution of each position.

Top K Frequent Words

Sat, 16 May 2026 00:00:00 GMT

Return the k most frequent words, ties broken lexicographically — counter + heap.

Transpose Matrix

Sat, 16 May 2026 00:00:00 GMT

Allocate a swapped-dimension grid and copy with indices flipped.

Trapping Rain Water

Sat, 16 May 2026 00:00:00 GMT

Two pointers — water level on each side is bounded by its taller wall so far; advance the shorter side.

Tree Diameter

Sat, 16 May 2026 00:00:00 GMT

Two BFS passes — first from any node finds a diameter endpoint; second BFS from there measures the diameter.

Triangle

Sat, 16 May 2026 00:00:00 GMT

Minimum top-to-bottom path sum in a triangle — in-place bottom-up DP.

Triples with Bitwise AND Equal To Zero

Sat, 16 May 2026 00:00:00 GMT

Pair-AND frequency lookup — for each i, count j,k with (a[j] & a[k] & a[i]) == 0 via a precomputed pairwise table.

Two City Scheduling

Sat, 16 May 2026 00:00:00 GMT

Send 2n people half to A, half to B minimizing total cost — sort by cost difference.

Two Sum III - Data structure design

Sat, 16 May 2026 00:00:00 GMT

add/find with frequency map — find scans entries and checks complement, doubling check for x+x = target.

Two Sum IV - Input is a BST

Sat, 16 May 2026 00:00:00 GMT

Does the BST contain two distinct nodes summing to k? Inorder + two-pointer or hash-set traversal.

Two Sum Less Than K

Sat, 16 May 2026 00:00:00 GMT

Sort and two-pointer for the maximum pair sum strictly less than k.

Two Sum

Sat, 16 May 2026 00:00:00 GMT

One-pass hash map of value to index — look up the complement before inserting the current value.

Unique Number of Occurrences

Sat, 16 May 2026 00:00:00 GMT

Check whether every value's occurrence count in the array is itself unique.

Unique Paths

Sat, 16 May 2026 00:00:00 GMT

1D DP rolling — each cell is the sum of the cell above and to its left.

Unique Paths III

Sat, 16 May 2026 00:00:00 GMT

Count paths from start to end visiting every empty cell exactly once — DFS with bitmask/count tracking.

Univalued Binary Tree

Sat, 16 May 2026 00:00:00 GMT

DFS verifying every node's value matches the root's.

Valid Palindrome II

Sat, 16 May 2026 00:00:00 GMT

Allow up to one character deletion — return true if the string can be made a palindrome.

Valid Anagram

Sat, 16 May 2026 00:00:00 GMT

Check whether two strings are anagrams — letter-count comparison.

Valid Parentheses

Sat, 16 May 2026 00:00:00 GMT

Push openers; on a closer, the stack top must be its match.

Valid Palindrome

Sat, 16 May 2026 00:00:00 GMT

Check if a string reads the same forward and backward after lowercasing and dropping non-alphanumerics.

Valid Sudoku

Sat, 16 May 2026 00:00:00 GMT

Verify a partially filled Sudoku board obeys row, column, and 3x3 box rules.

Valid Square

Sat, 16 May 2026 00:00:00 GMT

Compute all 6 pairwise squared distances — a square has exactly 4 equal smaller and 2 equal larger, with the diagonal = side * sqrt(2).

Valid Triangle Number

Sat, 16 May 2026 00:00:00 GMT

Sort, then for each largest side use two pointers to count valid (a, b) pairs.

Validate Binary Search Tree

Sat, 16 May 2026 00:00:00 GMT

DFS passes a (min, max) bound interval — each node's value must lie strictly inside.

Valid Word Abbreviation

Sat, 16 May 2026 00:00:00 GMT

Check if a word matches an abbreviation that interleaves letters with digit counts. Two pointers, no leading zeros.

Vertical Order Traversal of a Binary Tree

Sat, 16 May 2026 00:00:00 GMT

Group nodes by column, breaking ties by row then by value.

Verifying an Alien Dictionary

Sat, 16 May 2026 00:00:00 GMT

Are the words sorted in the given alien order? Pairwise compare via rank map.

Water and Jug Problem

Sat, 16 May 2026 00:00:00 GMT

Solvable iff target ≤ x + y and target is a multiple of gcd(x, y) — Bezout's identity.

Vowel Spellchecker

Sat, 16 May 2026 00:00:00 GMT

Look up queries against a wordlist with three fallback levels — exact, case-insensitive, vowel-mask.

Where Will the Ball Fall

Sat, 16 May 2026 00:00:00 GMT

Simulate each ball top-to-bottom, redirecting via the diagonal sign and detecting wall-stuck cases.

Wildcard Matching

Sat, 16 May 2026 00:00:00 GMT

Match s against pattern p with `?` and `*` wildcards — greedy two-pointer with backtrack on mismatch.

Word Break II

Sat, 16 May 2026 00:00:00 GMT

Return all sentences s can be segmented into — memoized DFS keyed by start index.

Word Break

Sat, 16 May 2026 00:00:00 GMT

Can s be segmented into dictionary words? 1D DP over prefixes.

Word Pattern

Sat, 16 May 2026 00:00:00 GMT

Check whether a pattern's characters bijectively match words in a sentence.

Word Ladder

Sat, 16 May 2026 00:00:00 GMT

Shortest transformation sequence from begin to end via single-letter edits — BFS over a word graph.

Word Ladder II

Sat, 16 May 2026 00:00:00 GMT

All shortest transformation sequences from begin to end — BFS to compute layered distances, DFS to enumerate paths.

Word Search II

Sat, 16 May 2026 00:00:00 GMT

Find every dictionary word in a board — trie-pruned DFS over the grid.

Word Search

Sat, 16 May 2026 00:00:00 GMT

Find a word in a 2D grid via DFS with in-place visited marking.