Letter Combinations of a Phone Number

    public ArrayList<String> letterCombinations(String digits) {        // Start typing your Java solution below        // DO NOT write main() function        String[] combination = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};        return letterCombinations(digits, 0, combination);    }        public ArrayList<String> letterCombinations(String digits, int index, String[] combination) {        ArrayList<String> result = new ArrayList<String>();        if(index == digits.length()) {            result.add("");            return result;        }else {            String tmp = combination[digits.charAt(index) - '0'];            for(int i = 0; i < tmp.length(); i++){                for(String s : letterCombinations(digits, index + 1, combination)) {                    StringBuilder sb = new StringBuilder();                    sb.append(tmp.charAt(i));                    sb.append(s);                    result.add(sb.toString());                }            }        }        return result;    }