leetcode-230 二叉搜索树中第 K 小的元素

/**
 * 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}
 */
var kthSmallest = function (root, k) {
  // 中序遍历
  let count = k,
    result = null;
  function dfs(node) {
    if (!node) return;
    dfs(node.left);
    count--;
    if (count === 0) {
      result = node.val;
      return;
    }
    if (result!==null) {
      return;
    }
    dfs(node.right);
  }
  dfs(root);
  return result;
};