private:
   struct node {
      std::string elem;
      node *next;
   }
   node *head, *tail;
}

string Queue::dequeue()  {
   if(head==NULL)
      error('Dequeing an empty queue")
   string str=head->elem;
   node *next = head->next;
   delete head;
   head=next;
   if(head==NULL) tail=NULL;
   return str;
}