[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

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class Solution {
     int res;
     int count;
     public int kthSmallest(TreeNode root, int k) {
         res = 0;
         count = k;
         inOrder(root);
         return res;
     }

     private void inOrder(TreeNode root) {
         if (root == null) {
             return;
         }
         inOrder(root.left);
         count -= 1;
         if (count == 0) {
             res = root.val;
             return;
         }
         inOrder(root.right);
     }
 }