/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode ret;
    private int cnt = 0;
    //70. 二叉搜索树的第k个结点
    public TreeNode kthNode(TreeNode root, int k) {
        inOrder(root, k);
        return ret;
    }

    private void inOrder(TreeNode root, int k) {
        if (root == null || cnt >= k) {
            return;
        }

        inOrder(root.left, k);
        cnt++;
        if (cnt == k) {
            ret = root;
        }
        inOrder(root.right, k);
    }
}