98. Validate Binary Search Tree | LeetCode Solution
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
/**
* 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:
int rootVal;
bool helper(TreeNode *root, TreeNode *minNode, TreeNode *maxNode)
{
if (!root)
return true;
if (minNode && root->val <= minNode->val || maxNode && root->val >= maxNode->val)
return false;
return helper(root->left, minNode, root) && helper(root->right, root, maxNode);
}
bool isValidBST(TreeNode *root)
{
return helper(root, NULL, NULL);
}
};
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