题目链接：二叉搜索树的第k个结点

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param proot TreeNode类 
     * @param k int整型 
     * @return int整型
     */

    TreeNode res = new TreeNode(-1);
    int count = 0;
    public int KthNode (TreeNode root, int k) {
        // 二叉搜索树：中序遍历为递增序列的二叉树
        if(root == null || k <= 0) return -1;
        this.count = k;
        dfs(root);
        return res.val;
    }

    public void dfs(TreeNode root){
        if(root == null) return;
        dfs(root.left);
        count --;
        if(count == 0){
             res = root;
             return;
        }
        if(count > 0) dfs(root.right);

    }

}