# Validate Binary Search Tree Problem

## Description

LeetCode Problem 98.

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:

``````1
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Input: root = [2,1,3]
Output: true
``````

Example 2:

``````1
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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, 10^4].
• -2^31 <= Node.val <= 2^31 - 1

## Sample C++ Code

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class Solution {
public:
bool isValidBST(TreeNode* root) {
TreeNode* prev = NULL;
return validate(root, prev);
}
bool validate(TreeNode* node, TreeNode* &prev) {
if (node == NULL) return true;
if (!validate(node->left, prev)) return false;
if (prev != NULL && prev->val >= node->val) return false;
prev = node;
return validate(node->right, prev);
}
};
``````