Kth Smallest Element in a BST

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        stack<TreeNode*> st;
        TreeNode* cur = root;
        while (cur || !st.empty()) {
            while (cur) { st.push(cur); cur = cur->left; }
            cur = st.top(); st.pop();
            if (--k == 0) return cur->val;
            cur = cur->right;
        }
        return -1;
    }
};

func kthSmallest(root *TreeNode, k int) int {
    st := []*TreeNode{}
    cur := root
    for cur != nil || len(st) > 0 {
        for cur != nil {
            st = append(st, cur)
            cur = cur.Left
        }
        cur = st[len(st)-1]
        st = st[:len(st)-1]
        k--
        if k == 0 {
            return cur.Val
        }
        cur = cur.Right
    }
    return -1
}

from typing import Optional

class Solution:
    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
        st = []
        cur = root
        while cur or st:
            while cur:
                st.append(cur)
                cur = cur.left
            cur = st.pop()
            k -= 1
            if k == 0:
                return cur.val
            cur = cur.right
        return -1

function kthSmallest(root, k) {
    const st = [];
    let cur = root;
    while (cur || st.length) {
        while (cur) {
            st.push(cur);
            cur = cur.left;
        }
        cur = st.pop();
        if (--k === 0) return cur.val;
        cur = cur.right;
    }
    return -1;
}

class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Deque<TreeNode> st = new ArrayDeque<>();
        TreeNode cur = root;
        while (cur != null || !st.isEmpty()) {
            while (cur != null) {
                st.push(cur);
                cur = cur.left;
            }
            cur = st.pop();
            if (--k == 0) return cur.val;
            cur = cur.right;
        }
        return -1;
    }
}

function kthSmallest(root: TreeNode | null, k: number): number {
    const st: TreeNode[] = [];
    let cur: TreeNode | null = root;
    while (cur || st.length) {
        while (cur) {
            st.push(cur);
            cur = cur.left;
        }
        cur = st.pop()!;
        if (--k === 0) return cur.val;
        cur = cur.right;
    }
    return -1;
}