public Set<Coordinates> validate() {
        Set<Coordinates> fails = new HashSet<Coordinates>();
        Set<Integer> alreadyThere = new HashSet<Integer>();
        List<Integer> twoTimes = new ArrayList<Integer>();

        // check lines
        for (int x = 0; x < board.length; x++) {
            for (int y = 0; y < board.length; y++) {
                int value = board[x][y];
                if (value > 0 && value <= size && !alreadyThere.contains(value)) {
                    alreadyThere.add(value);
                }
                else if(value != 0) {
                    twoTimes.add(value);
                }
            }
            for (int i = 0; i < twoTimes.size(); i++) {
                int ban = twoTimes.get(i);
                for (int j = 0; j < board.length; j++) {
                    if (board[x][j] == ban) {
                        fails.add(new Coordinates(x, j));
                    }
                }
            }
            alreadyThere.clear();
        }
        twoTimes.clear();

        // check columns
        for (int y = 0; y < board.length; y++) {
            for (int x = 0; x < board.length; x++) {
                int value = board[x][y];
                if (value > 0 && value <= size && !alreadyThere.contains(value)) {
                    alreadyThere.add(value);
                }
                else if(value != 0) {
                    twoTimes.add(value);
                }
            }
            for (int i = 0; i < twoTimes.size(); i++) {
                int ban = twoTimes.get(i);
                for (int j = 0; j < board.length; j++) {
                    if (board[j][y] == ban) {
                        fails.add(new Coordinates(j, y));
                    }
                }
            }
            alreadyThere.clear();
        }
        return fails;
    }