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.

Validate Binary Search Tree.cpp

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return valid(root, numeric_limits<int>::min(), numeric_limits<int>::max());
    }
    
    
    bool valid(TreeNode* root, int left, int right) {
        if(root == NULL) {
            return true;
        } else if(root->val >= right or root->val <= left) {
            return false;
        } else {
            return valid(root->left, left, root->val) and
                    valid(root->right, root->val, right);
        }
    }
    
    
};