LeetCode 230.二叉搜索树中第K小的元素 - JavaScript

// ac地址：https://leetcode-cn.com/problems/kth-smallest-element-in-a-bst/
// 原文地址：https://xxoo521.com/2020-03-05-kth-smallest-element-in-a-bst/
/**
 * @param {TreeNode} root
 * @param {number} k
 * @return {number}
 */
var kthSmallest = function(root, k) {
    let i = 0;
    let val = null;
    travel(root);
    return val;

    function travel(node) {
        node.left && travel(node.left);

        if (++i === k) {
            val = node.val;
            return;
        }

        node.right && travel(node.right);
    }
};