[LC] 230. 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.

Example 1:

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

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
       5
      / \
     3   6
    / \
   2   4
  /
 1
Output: 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 class Solution {
11     int res;
12     int count;
13     public int kthSmallest(TreeNode root, int k) {
14         res = 0;
15         count = k;
16         inOrder(root);
17         return res;
18     }
19     
20     private void inOrder(TreeNode root) {
21         if (root == null) {
22             return;
23         }
24         inOrder(root.left);
25         count -= 1;
26         if (count == 0) {
27             res = root.val;
28             return;
29         }
30         inOrder(root.right);
31     }
32 }