230. Kth Smallest Element in a BST

1/**
2 * Definition for a binary tree node.
3 * class TreeNode {
4 *     val: number
5 *     left: TreeNode | null
6 *     right: TreeNode | null
7 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
8 *         this.val = (val===undefined ? 0 : val)
9 *         this.left = (left===undefined ? null : left)
10 *         this.right = (right===undefined ? null : right)
11 *     }
12 * }
13 */
14
15function kthSmallest(root: TreeNode | null, k: number): number {
16    let res = -1;
17    let count = k;
18    function dfs(node: TreeNode | null): void {
19        if (!node) return;
20        dfs(node.left);
21        count--
22        if (count === 0) {
23            res = node.val
24            return
25        }
26        dfs(node.right)
27    }
28    dfs(root)
29
30    return res;
31};