leetcode------Validate Binary Search Tree

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? > read more on how binary tree is serialized on OJ.

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：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return isBst(root,Long.MIN_VALUE,Long.MAX_VALUE);
    }
    public boolean isBst(TreeNode root,long min,long max){
    if(root==null) return true;
    if(root.val<=min||root.val>=max)return false;
    return isBst(root.left,min,root.val)&&isBst(root.right,root.val,max);
       
    }
}