98. Validate Binary Search Tree

https://leetcode.com/problems/validate-binary-search-tree/

Problem

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.

Solution

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        if (root==nullptr) return true;
        return isValidBST(root->left, &root->val, nullptr) && isValidBST(root->right, nullptr, &root->val);
    }
    bool isValidBST(TreeNode* root, int* max, int* min){
        if (root == nullptr) return true;
        if (max != nullptr && root->val >= *max) return false;
        if (min != nullptr && root->val <= *min) return false;
        return isValidBST(root->left, &root->val, min) && isValidBST(root->right, max, &root->val);
    }
};

• #tree

• #bst