@Test
    public void returnListOfStrings_whenCallPermuteAsStrings() {
        List<Character> chars = new ArrayList<Character>(Arrays.asList('a', 'b', 'c'));
        String expected = "[abc, acb, bac, bca, cba, cab]";
        String actual = permutation.permuteAsStrings(chars).toString();
        assertEquals(expected, actual);

    }