[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);
    }
}