void remove(const T& el) {
  while(head != nullptr && head->data == el) {
    node<T>* toDelete = head;
    head = head->next;
    delete toDelete;
  }
  if(head == nullptr)
    tail = nullptr;
  node<T>* previous = head;
  while(previous != nullptr) {
    if(previous->next != nullptr&&previous->next->data == el) {
      node<T>* toDelete = previous->next;
      previous->next = previous->next->next;
      if(previous->next == nullptr)
        tail = previous;
      delete toDelete;
    } else
      previous = previous->next;
  }
}