[LeetCode][JavaScript]Kth Smallest Element in a BST

Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note: 
You may assume k is always valid, 1 ≤ k ≤ BST‘s total elements.

Follow up:

What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

Hint:

1. Try to utilize the property of a BST.
2. What if you could modify the BST node‘s structure?
3. The optimal runtime complexity is O(height of BST).

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

二叉搜索树的特性，先序遍历的输出就是排序的结果。

 1 /**
 2  * Definition for a binary tree node.
 3  * function TreeNode(val) {
 4  *     this.val = val;
 5  *     this.left = this.right = null;
 6  * }
 7  */
 8 /**
 9  * @param {TreeNode} root
10  * @param {number} k
11  * @return {number}
12  */
13 var kthSmallest = function(root, k) {
14     var count = 0;
15     var isFound = false;
16     var res = null;
17     inorder(root);
18     return res;
19
20     function inorder(node){
21         if(node !== null && !isFound){
22             inorder(node.left);
23             count++;
24             if(count === k){
25                 res = node.val;
26                 isFound = true;
27                 return;
28             }
29             inorder(node.right);
30         }
31     }
32 };