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 = 1   3  / \ 1   4  \   2Output: 1

Example 2:

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

1 /** 2  * Definition for a binary tree node. 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public int kthSmallest(TreeNode root, int k) {12         int[] arr = new int[2];13         helper(root, arr, k);14         return arr[1];15     }16     17     public void helper(TreeNode root, int[] arr, int k) {18         if (root != null) {19             helper(root.left, arr, k);20             arr[0]++;21             if (arr[0] == k) {22                 arr[1] = root.val;23                 return;24             }25             helper(root.right, arr, k);26         }27     }28 }