Leetcode: Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

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

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

Solution:

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        ArrayList<String> previous = new ArrayList<String>();
        previous.add("");
        ArrayList<String> actual;
        for(int i=0; i<digits.length(); i++)
        {
            actual = new ArrayList<String>();
            for(String s: previous)
            {
                switch (digits.charAt(i))
                {
                    case '2': actual.add(s+"a"); actual.add(s+"b");
                        actual.add(s+"c"); break;
                    case '3': actual.add(s+"d"); actual.add(s+"e");
                        actual.add(s+"f"); break;
                    case '4': actual.add(s+"g"); actual.add(s+"h");
                        actual.add(s+"i"); break;
                    case '5': actual.add(s+"j"); actual.add(s+"k");
                        actual.add(s+"l"); break;
                    case '6': actual.add(s+"m"); actual.add(s+"n");
                        actual.add(s+"o"); break;
                    case '7': actual.add(s+"p"); actual.add(s+"q");
                        actual.add(s+"r"); actual.add(s+"s"); break;
                    case '8': actual.add(s+"t"); actual.add(s+"u");
                        actual.add(s+"v"); break;
                    case '9': actual.add(s+"w"); actual.add(s+"x");
                        actual.add(s+"y"); actual.add(s+"z"); break;
                }
            }
            previous = actual;
        }
        return previous;
    }
}