LeetCode-in-All
98. Validate Binary Search Tree
Medium
Given the root of a binary tree, determine if it is a valid binary search tree (BST).
A valid 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:
Input: root = [2,1,3]
Output: true
Example 2:
Input: root = [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.
Constraints:
- The number of nodes in the tree is in the range
[1, 104]. -231 <= Node.val <= 231 - 1
Solution
#include <stdio.h>
#include <stdbool.h>
#include <limits.h>
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* struct TreeNode *left;
* struct TreeNode *right;
* };
*/
// Helper function to check if the tree is a valid BST
bool solve(struct TreeNode* root, long left, long right) {
if (root == NULL) {
return true;
}
if (root->val <= left || root->val >= right) {
return false;
}
return solve(root->left, left, root->val) && solve(root->right, root->val, right);
}
// Main function to check if a tree is a valid BST
bool isValidBST(struct TreeNode* root) {
return solve(root, LONG_MIN, LONG_MAX);
}