public int NumShips(int playerID) {
        int numShips = 0;
        for (Planet p : planets) {
            if (p.Owner() == playerID) {
                numShips += p.NumShips();
            }
        }
        for (Fleet f : fleets) {
            if (f.Owner() == playerID) {
                numShips += f.NumShips();
            }
        }
        return numShips;
    }