Node * insert(const Key& k, const Value& v, Node *rt) {
    if (rt == NULL) {
      rt = new Node(k,v);
    } else if (k < rt->key) {
      rt->left = insert(k, v, rt->left);
    } else if (k > rt->key) {
      rt->right = insert(k, v, rt->right);
    }
    return rt;
  }