题解 | #判断是不是二叉搜索树#

using System;
using System.Collections.Generic;

/*
public class TreeNode
{
	public int val;
	public TreeNode left;
	public TreeNode right;

	public TreeNode (int x)
	{
		val = x;
	}
}
*/

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 root TreeNode类 
     * @return bool布尔型
     */
    bool isFirst = true;
    int pre = 0;
    public bool isValidBST(TreeNode root) {
        if(root == null) return true;
        if(!isValidBST(root.left)) return false;
        if(!isFirst) {if(root.val <= pre) return false;}
        else isFirst = !isFirst;
        pre = root.val;
        return isValidBST(root.right);
    } 
}