Validate Binary Search Tree

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.

/** * 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) {        return isChildBST(root,NULL,NULL);    }    bool isChildBST(TreeNode *root, TreeNode *l, TreeNode *r){        if(root == NULL){            return true;        }        if(r != NULL && root->val >= r->val){            return false;        }        if(l != NULL && root->val <= l->val){            return false;        }        bool bl = isChildBST(root->left, l, root) && isChildBST(root->right, root, r);        return bl;    }};