public class BookingSystem {
  private ArrayList<Boolean> reservations;

  public BookingSystem() {

    reservations = new ArrayList<Boolean>(24); 

    for (int i = 0; i < 24; i++){
      reservations.add(Boolean.FALSE);
    }
  }

  public List<Integer> listBookedHours() {
    List<Integer> booked = new ArrayList<Integer>(); 
    for (int i = 0; i < 24; i++){
      if (reservations.get(i)) {
        booked.add(i); 
      }
    }
    return booked; 
  }

  public boolean reserve(int hour) {
    if (hour < 0 || hour > 23){
      throw new IllegalArgumentException();
    }

    if (reservations.get(hour)){ return false;}

    else {
      reservations.add(hour, Boolean.TRUE);
      return true;
    }
  }
}