private static boolean isSectorCorrect(final int[][] solution) {
        BitSet numbers = new BitSet();
        for (int a = 0; a < FIELD_SIZE; a += SECTOR_SIZE) {
            for (int b = 0; b < FIELD_SIZE; b += SECTOR_SIZE) {
                for (int i = a; i < a + SECTOR_SIZE; ++i) {
                    for (int j = b; j < b + SECTOR_SIZE; ++j) {
                        numbers.set(solution[i][j]);
                    }
                }

                if (numbers.cardinality() < FIELD_SIZE) {
                    return false;
                }
                numbers.clear();
            }
        }

        return true;
    }