230. Kth Smallest Element in a BST

https://leetcode.com/problems/kth-smallest-element-in-a-bst/

time: O(n)

space: O(h)

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */

function Track() {
    this.val;
    this.cnt = 0;
}

let track;
var kthSmallest = function (root, k) {
    track = new Track();
    find(root, k);
    return track.val;
};

function find(node, k) {
    if (node === null) return;
    if (track.val !== undefined) return;
    
    find(node.left, k);
    
    track.cnt++;
    if (track.cnt === k) {
        track.val = node.val;
        return;
    }


    find(node.right, k);
}