[leetcode] 98. Validate Binary Search Tree

[leetcode] 98. Validate Binary Search Tree

Description

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 1:

2 / \ 1 3

Input:

[2,1,3]

Output:

true

Example 2:

5 / \ 1 4 / \ 3 6

Input:

[5,1,4,null,null,3,6]

Output:

false

Explanation:

The root node's value is 5 but its right child's value is 4.

分析

题目的意思是：判断一颗二叉树是否是合法的二叉搜索树。

对于一棵二叉搜索树，根结点必须比所有的左子树的值大，然后比右子树的所有的值小。这样就很好理解为什么要用min,max指针了。注意数的溢出问题。

代码

/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public: bool isValidBST(TreeNode* root) { return judge(root,LONG_MIN,LONG_MAX); } bool judge(TreeNode *root, long min_val,long max_val){ if(root==NULL){ return true; } if(root->val<=min_val||root->val>=max_val){ return false; } return judge(root->left,min_val,root->val)&&judge(root->right,root->val,max_val); }};

参考文献

​​[编程题]validate-binary-search-tree​​​​[LeetCode] Validate Binary Search Tree 验证二叉搜索树​​

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