public List<String> letterCasePermutation(String s) {
        if (s == null || s.length() == 0)
            return new ArrayList<>();
        List<String> res = new ArrayList<>();
        res.add(s.toLowerCase());
        go(s.toLowerCase().toCharArray(), 0, res);
        return res;
    }
Why Did you create this class? what does it do?
src/problems/easy/LetterCasePermutation.java