标题	Validate Binary Search Tree
通过率	21.9%
难度	中等

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

 

confused what "{1,#,2,3}" means? 

本题就是确定一棵树是否为二叉排序树，左<中<右，可以用中序遍历去判断，也可以用一个范围值去判断当前节点是否在其中，范围的下界是左边的树，范围的右界是右边的树，直接看代码了：

1 /** 2  * Definition for binary tree 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public boolean isValidBST(TreeNode root) {12         return isBst(root,Long.MIN_VALUE,Long.MAX_VALUE);13     }14     public boolean isBst(TreeNode root,long min,long max){15     if(root==null) return true;16     if(root.val<=min||root.val>=max)return false;17     return isBst(root.left,min,root.val)&&isBst(root.right,root.val,max);18        19     }20 }