Queue::Queue()
/*
Post: The Queue is initialized to be empty.
*/
{
   front = rear = NULL;
}


Error_code Queue::append(const Queue_entry &item)
/*
Post: Add item to the rear of the Queue and return a code of success
      or return a code of overflow if dynamic memory is exhausted.
*/
{
  Node *new_rear = new Node(item);
  if (new_rear == NULL) return overflow;
  if (rear == NULL) front = rear = new_rear;
  else {
    rear->next = new_rear;
    rear = new_rear;
  }
  return success;
}


Error_code Queue::serve()
/*
Post: The front of the Queue is removed.  If the Queue
      is empty, return an Error_code of underflow.
*/
{
   if (front == NULL) return underflow;
   Node *old_front = front;
   front = old_front->next;
   if (front == NULL) rear = NULL;
   delete old_front;
   return success;
}