Validate_Binary_Search_Tree.cpp

/*
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.
*/

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