public int Winner() {
        Set<Integer> remainingPlayers = new TreeSet<Integer>();
        for (Planet p : planets) {
            remainingPlayers.add(p.Owner());
        }
        for (Fleet f : fleets) {
            remainingPlayers.add(f.Owner());
        }
        switch (remainingPlayers.size()) {
        case 0:
            return 0;
        case 1:
            return ((Integer)remainingPlayers.toArray()[0]).intValue();
        default:
            return -1;
        }
    }