LC 98. Validate Binary Search Tree

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 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.

参考答案

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return foo(root,NULL,NULL);
    }
    bool foo(TreeNode* root,TreeNode* minNode,TreeNode* maxNode){
        if(!root) return true;
        if(minNode && root->val <= minNode->val || maxNode && root->val >= maxNode->val) return false;
        return foo(root->left,minNode,root) && foo(root->right,root,maxNode);
    }
};

答案分析