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?

難度：medium

題目：給定二叉搜索樹，找出其值第K小的結點。

思路：中序遍歷

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);} }

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