题解 | 二叉搜索树的第k个结点

树结构：

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

二叉搜索树的中序序列是升序序列：

import java.util.*;
public class Solution {
    public TreeNode KthNode(TreeNode root, int k) {
        List<TreeNode> l = new ArrayList<>();
        inOrder(root, l);
        if (k > l.size() || k < 1) 
            return null;
        else 
            return l.get(k-1);
    }

    // 递归辅助函数
    private void inOrder(TreeNode root, List<TreeNode> list) {
        if (root != null) {
            inOrder(root.left, list);
            list.add(root);
            inOrder(root.right, list);
        }
    }
}