is_binary_search_tree

int is_binary_search_tree(struct node *n) {
  return is_bst(n, INT_MIN, INT_MAX);
}

int is_bst(struct node *n, int min, int max) {
  return !n || (n >= min && n < max) && is_bst(n->left, min, n->val) && is_bst(n->right, n->val, max);
}