【Java】letter combinations of phone number

/**
* Created on 16/6/10.
*/
import java.util.InputMismatchException;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
public class letterCombinationsOfPhoneNumber {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String x = in.next();
System.out.println(letterCombinationsOfPhoneNumber(x));
System.out.println(letterCombinations(x));
}

public static ArrayList<String> letterCombinations(String digits) {
HashMap<Character, String> map = new HashMap<Character, String>(){{
put('2', "abc"); put('3', "def"); put('4', "ghi"); put('5', "jkl");
put('6', "mno"); put('7', "pqrs"); put('8', "tuv"); put('9', "wxyz");
}};

ArrayList<String> ret = new ArrayList<>();
if (digits.equals("")) {
return ret;
}
dfs(digits, 0, "", map, ret);
return ret;
}

public static void dfs(String digits, int index, String path, HashMap<Character, String> map, List<String> ret) {
if (digits.length() == path.length()) {
ret.add(path);
return;
}
for (int i = index; i < digits.length(); i++) {
for (char c : map.get(digits.charAt(i)).toCharArray()) {
dfs(digits, i + 1, path + c, map, ret);
}
}
}

public static ArrayList<String> letterCombinationsOfPhoneNumber(String digits) {
HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put(2, "abc");
map.put(3, "def");
map.put(4, "ghi");
map.put(5, "jkl");
map.put(6, "mno");
map.put(7, "pqrs");
map.put(8, "tuv");
map.put(9, "wxyz");
map.put(0, "");
ArrayList<String> result = new ArrayList<>();
if (digits == null || digits.length() == 0) {
return result;
}
ArrayList<Character> tmp = new ArrayList<>();
getString(digits, tmp, result, map);
return result;
}

public static void getString(String digits, ArrayList<Character> tmp, ArrayList<String> result, HashMap<Integer, String> map) {
if (digits.length() == 0) {
char[] arr = new char[tmp.size()];
for (int i = 0; i < tmp.size(); i++) {
arr[i] = tmp.get(i);
}
result.add(String.valueOf(arr));
return;
}
Integer curr = Integer.valueOf(digits.substring(0, 1));
String letters = map.get(curr);
for (int i = 0; i < letters.length(); i++) {
tmp.add(letters.charAt(i));
getString(digits.substring(1), tmp, result, map);
tmp.remove(tmp.size() - 1);
}
}

}