private void appendDigits(String digits,int digitIndex,List<String> list,StringBuffer letter){
        if(digitIndex == digits.length()){
            list.add(letter.toString());
            return;
        }
        int index= digits.charAt(digitIndex)-48;
        String buttonString = board[index];
        System.out.println(buttonString);

        for(int i=0;i<buttonString.length();i++){
            letter.append(buttonString.charAt(i));
            appendDigits(digits,digitIndex+1,list,letter);
        }

    }