template <class T>
void QueueList<T>::enQueue(const T& e)
{
  if (tail_ == nullptr)
  {
    tail_ = new Node;
    head_ = tail_;
    tail_->value_ = e;
    tail_->next_ = nullptr;
  }
  else
  {
    Node* newItem = new Node;
    newItem->value_ = e;
    newItem->next_ = nullptr;
    tail_->next_ = newItem;
    tail_ = newItem;
  }
}