leetcode98 - Validate Binary Search Tree - medium

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.

 

对于每个node，都有一个限制的range，以此进行递归。node为null是返回的是true

注意的是这里取INT_LIMITS会out of range, 用long就好

 

实现：O(n), 访问每个node exactly once

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return helper(root, LONG_MIN, LONG_MAX);
    }
    
    bool helper(TreeNode* root, long min, long max){
        if (!root) return true;
        if (root->val <= min || root->val >= max) return false;
        return helper(root->left, min, root->val) && helper(root->right, root->val, max);
    }
};