@Test
    public void combinationWithLength_shouldReturnCombOfMaxLengthK() {
        String word = "abc";
        assertEquals("[[a], [b], [c], [a, b], [a, c], [b, c]]",
                combination.combination(word,2).toString());
        assertEquals("[[a], [b], [c], [a, b], [a, c], [b, c], [a, b, c]]",
                combination.combination(word,3).toString());

    }