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.
Example 1:

Input: root = [3,1,4,null,2], k = 13/ \1   4\2
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 35/ \3   6/ \2   4/1
Output: 3

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?

难度：medium

题目：给定二叉搜索树，找出其值第K小的结点。

思路：中序遍历

Runtime: 0 ms, faster than 100.00% of Java online submissions for Kth Smallest Element in a BST.
Memory Usage: 38.9 MB, less than 19.71% of Java online submissions for Kth Smallest Element in a BST.

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode(int x) { val = x; }* }*/
public class Solution {public int kthSmallest(TreeNode root, int k) {int[] result = {root.val};kthSmallest(root, k, new int[1], result);return result[0];}public void kthSmallest(TreeNode root, int k, int[] count, int[] result) {if (root == null || count[0] >= k) {return;}kthSmallest(root.left, k, count, result);count[0]++;if (count[0] == k) {result[0] = root.val;return;} kthSmallest(root.right, k, count, result);}
}