bool is_leaf(TreeNode* root) {
    if(root == NULL) return 0;
    return (root->left == NULL) and (root->right == NULL);
}

// We are considering that BSTs can not contain duplicate Nodes.
bool check_bst(TreeNode *root) {
    if((root == NULL) or (is_leaf(root) == 1)) return 1;

    bool res = 1;
    if(root->left) res &= (root->left->val < root->val);
    if(root->right) res &= (root->right->val > root->val);

    return (res and check_bst(root->left) and check_bst(root->right));
}

/* One possible tree where the above algorithm will give wrong result is :--->

           5
          / \
         /   \
        4     6
             / \
            /   \
           3     7
*/