// if there are any empty rooms (rooms with no reservation),
  // then create a reservation for an empty room for the
  // specified guest and return the new Reservation;
  // otherwise, add the guest to the end of waitList
  // and return null
  public Reservation requestRoom(String guestName)
  {
    for (int i = 0; i < rooms.length; i++)
    {
      if (rooms[i] == null)
      {
        Reservation res = new Reservation(guestName, i);
        rooms[i] = res;
        return res; 1
      }
    }
    waitList.add(guestName);
    return null; 
  } 1

1. Don't forget to quit once an available room is found.
   
   
2. Also possible but slightly longer:

     Reservation res = null;
     for (int i = 0; i < rooms.length && res == null; i++)
       {
         if (rooms[i] == null)
         {
           res = new Reservation(guestName, i);
           rooms[i] = res;
         }
     }
     if (res == null)
       waitList.add(guestName);
     return res;

  // release the room associated with parameter res, effectively
  // cancelling the reservation;
  // if any names are stored in waitList, remove the first name
  // and create a Reservation for this person in the room 
  // reserved by res; return that new Reservation;
  // if waitList is empty, mark the room specified by res as empty and
  // return null
  // precondition:  res is a valid Reservation for some room
  //                in this hotel
  public Reservation cancelAndReassign(Reservation res)
  {
    Reservation newRes = null;
    int i = res.getRoomNumber();

    if (!waitList.isEmpty())
    {
      newRes = new Reservation((String)waitList.get(0), i);
      waitList.remove(0); 1
    }
    rooms[i] = newRes;
    return newRes;
  }

1. Or:

         newRes = new Reservation((String)waitList.remove(0), i);