How can I print out all possible letter combinations a given phone number can represent?

Question

I just tried for my first programming interview and one of the questions was to write a program that given a 7 digit telephone number, could print all possible combinations

Answer 1

static final String[] keypad = {"", "", "ABC", "DEF", "GHI", "JKL", "MNO", "PQRS", "TUV", "WXYZ"};



String[] printAlphabet(int num){
        if (num >= 0 && num < 10){
            String[] retStr;
            if (num == 0 || num ==1){
                retStr = new String[]{""};
            } else {
                retStr = new String[keypad[num].length()];
                for (int i = 0 ; i < keypad[num].length(); i++){
                    retStr[i] = String.valueOf(keypad[num].charAt(i));
                }
            }
            return retStr;
        }

        String[] nxtStr = printAlphabet(num/10);

        int digit = num % 10;

        String[] curStr = null;
        if(digit == 0 || digit == 1){
            curStr = new String[]{""};
        } else {
            curStr = new String[keypad[digit].length()];
            for (int i = 0; i < keypad[digit].length(); i++){
                curStr[i] = String.valueOf(keypad[digit].charAt(i));
            }
        }

        String[] result = new String[curStr.length * nxtStr.length];
        int k=0;

        for (String cStr : curStr){
            for (String nStr : nxtStr){
                result[k++] = nStr + cStr;
            }
        }
        return result;
    }