public void basic() {
        List<Character> chars = new ArrayList<Character>(Arrays.asList('a', 'b', 'c'));
        String actual = permutation.permute(chars).toString();
        String expected = "[[a, b, c], [a, c, b], [b, a, c], [b, c, a], [c, b, a], [c, a, b]]";
        assertEquals(expected, actual);
    }