template <class K, class V>
void AVLTree<K, V>::remove(Node*& subtree, const K& key)
{
    if (subtree == NULL)
        return;

    if (key < subtree->key) {
        remove(subtree->left, key);
        rebalance(subtree);
    } else if (key > subtree->key) {
        remove(subtree->right, key);
        rebalance(subtree);
    } else {
        if (subtree->left == NULL && subtree->right == NULL) {
            /* no-child remove */
            // your code here
            delete subtree;
            subtree = NULL;
        } else if (subtree->left != NULL && subtree->right != NULL) {
            /* two-child remove */
            // your code here
            Node * iop = subtree->left;
            swap(iop, subtree);
            if (iop->left == NULL && iop->right == NULL) {
                delete iop;
                iop = NULL;
            }
            else{
              if(iop->right !=NULL){
                Node * right = iop->right;
                delete iop;
                iop = right;
              }
              else{
                Node* left = iop->left;
                delete iop;
                iop = left;
              }
            }
            rebalance(iop);
        } else {
            /* one-child remove */
            // your code here
            if(subtree->right !=NULL){
              Node * right = subtree->right;
              delete subtree;
              subtree = right;
            }
            else{
              Node* left = subtree->left;
              delete subtree;
              subtree = left;
            }
        }
        rebalance(subtree);
    }
}