map digits

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Created by shahin on 2/21/18.
 */
class MapDigits {

    //Input:Digit string "23"
    //Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

    static Map<String, List<String>> digitMap = new HashMap<>();

    MapDigits() {
        digitMap.put("2", Arrays.asList(new String[] {"a", "b", "c"}));
        digitMap.put("3", Arrays.asList(new String[] {"d", "e", "f"}));
        digitMap.put("4", Arrays.asList(new String[] {"g", "h", "i"}));
        digitMap.put("5", Arrays.asList(new String[] {"j", "k", "l"}));
        digitMap.put("6", Arrays.asList(new String[] {"m", "n", "o"}));
        digitMap.put("7", Arrays.asList(new String[] {"p", "q", "r", "s"}));
        digitMap.put("8", Arrays.asList(new String[] {"t", "u", "v"}));
        digitMap.put("9", Arrays.asList(new String[] {"w", "x", "y", "z"}));
    }

    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        /*
        if(digits.length() == 0) {
            System.out.println("doin for zero");
            return result;
        }
        */

        if(digits.length() == 1) {
            System.out.print("one: ");
            List<String> one= digitMap.get(digits.substring(0, 1));
            System.out.println(one);
            return one;
        }

        List<String> intermediate = letterCombinations(digits.substring(1, digits.length()));
        //System.out.println(intermediate);

        for(String first : digitMap.get(digits.substring(0,1)))
            for(String rest : intermediate)
                result.add(first + rest);
        return result;
    }

    public static void main(String args[]) {
        MapDigits md = new MapDigits();
        List<String> letters = md.letterCombinations("23");
        System.out.print("Anser: ");
        System.out.println(letters);
    }


}