【LeetCode】Validate Binary Search Tree 解题报告

标签（空格分隔）： LeetCode

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题目地址：https://leetcode.com/problems/validate-binary-search-tree/#/description

题目描述：

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.

Example：

    2   / \  1   3Binary tree [2,1,3], return true.    1   / \  2   3  Binary tree [1,2,3], return false.

Ways

判断一棵树是不是BST，那么按照定义，左子树的值要在(min,mid)之间，右子树的值在(mid,max)之间，这个mid值并不是中位数而是当前节点的值。这么一想，就可以很快的判断是不是符合BST的定义了。

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

Date

2017 年 4 月 17 日