public void Combinations( List<String> list, String letters,String digits,int index){
        if(index <=0){
            list.add(letters);
            return;
        }
        int j = digits.charAt(digits.length() -index)-'0' -1;
        for(int i=0; i < map[j].length; i++){
            String lettersNew = letters + map[j][i];
            Combinations(list,lettersNew,digits,index-1);
        }
    }