Decode String

class Solution {
public:
    string decodeString(string s) {
        stack<int> counts;
        stack<string> prefixes;
        string cur;
        int k = 0;
        for (char c : s) {
            if (isdigit(c)) {
                k = k * 10 + (c - '0');
            } else if (c == '[') {
                counts.push(k);
                prefixes.push(cur);
                k = 0;
                cur.clear();
            } else if (c == ']') {
                int rep = counts.top(); counts.pop();
                string prev = prefixes.top(); prefixes.pop();
                while (rep--) prev += cur;
                cur = prev;
            } else {
                cur.push_back(c);
            }
        }
        return cur;
    }
};

import "strings"

func decodeString(s string) string {
    counts := []int{}
    prefixes := []string{}
    var cur strings.Builder
    k := 0
    for i := 0; i < len(s); i++ {
        c := s[i]
        switch {
        case c >= '0' && c <= '9':
            k = k*10 + int(c-'0')
        case c == '[':
            counts = append(counts, k)
            prefixes = append(prefixes, cur.String())
            k = 0
            cur.Reset()
        case c == ']':
            rep := counts[len(counts)-1]
            counts = counts[:len(counts)-1]
            prev := prefixes[len(prefixes)-1]
            prefixes = prefixes[:len(prefixes)-1]
            inner := cur.String()
            cur.Reset()
            cur.WriteString(prev)
            for j := 0; j < rep; j++ {
                cur.WriteString(inner)
            }
        default:
            cur.WriteByte(c)
        }
    }
    return cur.String()
}

class Solution:
    def decodeString(self, s: str) -> str:
        counts: list[int] = []
        prefixes: list[str] = []
        cur = []
        k = 0
        for c in s:
            if c.isdigit():
                k = k * 10 + int(c)
            elif c == '[':
                counts.append(k)
                prefixes.append(''.join(cur))
                k = 0
                cur = []
            elif c == ']':
                rep = counts.pop()
                prev = prefixes.pop()
                inner = ''.join(cur)
                cur = [prev + inner * rep]
            else:
                cur.append(c)
        return ''.join(cur)

function decodeString(s) {
    const counts = [];
    const prefixes = [];
    let cur = '';
    let k = 0;
    for (const c of s) {
        if (c >= '0' && c <= '9') {
            k = k * 10 + (c.charCodeAt(0) - 48);
        } else if (c === '[') {
            counts.push(k);
            prefixes.push(cur);
            k = 0;
            cur = '';
        } else if (c === ']') {
            const rep = counts.pop();
            const prev = prefixes.pop();
            cur = prev + cur.repeat(rep);
        } else {
            cur += c;
        }
    }
    return cur;
}

class Solution {
    public String decodeString(String s) {
        Deque<Integer> counts = new ArrayDeque<>();
        Deque<StringBuilder> prefixes = new ArrayDeque<>();
        StringBuilder cur = new StringBuilder();
        int k = 0;
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isDigit(c)) {
                k = k * 10 + (c - '0');
            } else if (c == '[') {
                counts.push(k);
                prefixes.push(cur);
                k = 0;
                cur = new StringBuilder();
            } else if (c == ']') {
                int rep = counts.pop();
                StringBuilder prev = prefixes.pop();
                for (int j = 0; j < rep; j++) prev.append(cur);
                cur = prev;
            } else {
                cur.append(c);
            }
        }
        return cur.toString();
    }
}

function decodeString(s: string): string {
    const counts: number[] = [];
    const prefixes: string[] = [];
    let cur = '';
    let k = 0;
    for (const c of s) {
        if (c >= '0' && c <= '9') {
            k = k * 10 + (c.charCodeAt(0) - 48);
        } else if (c === '[') {
            counts.push(k);
            prefixes.push(cur);
            k = 0;
            cur = '';
        } else if (c === ']') {
            const rep = counts.pop() as number;
            const prev = prefixes.pop() as string;
            cur = prev + cur.repeat(rep);
        } else {
            cur += c;
        }
    }
    return cur;
}