Leetcode: 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 islessthanroot(TreeNode* left, int val)
{
if(left==NULL)return true;
if(left->val >= val)return false;
return islessthanroot(left->left,val) && islessthanroot(left->right,val);
}
bool islargethanroot(TreeNode* right, int val)
{
if(right==NULL)return true;
if(right->val <= val)return false;
return islargethanroot(right->left,val) && islargethanroot(right->right,val);
}
bool isValidBST(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(root == NULL) return true;
if(islessthanroot(root->left,root->val) && islargethanroot(root->right,root->val))
{
return isValidBST(root->left) && isValidBST(root->right);
}else
return false;

}
};