template <class Key, class Value>
struct Node
{
public:
    Key key;
    vector<Value> value;
    Node* leftNode;
    Node* rightNode;

    Node(Key k, Value v)
    {
        key = k;
        value.push_back(v);
    }
};

template <class Key, class Value>
void BinarySearchTree<Key, Value>::insert(const Key &key, const Value &value)
{
    insert(root, key, value);
}


template <class Key, class Value>
void BinarySearchTree<Key, Value>::insert(Node<Key, Value> *&node, const Key &key, const Value &value)
{
    if (node == nullptr)
        node = new Node<Key, Value>(key, value);

    // Error occurs here.
    else if (node->key == key)
        node->value.push_back(value);

    else if (key < node->key)
        insert(node->leftNode, key, value);
    else
        insert(node->rightNode, key, value);
}