Letter Combinations of a Phone Number

recursion problem

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<String> result = new ArrayList<String>();
        if(digits == null||digits.length() == 0)
        {
            result.add("");
            return result;
        }
        if(digits.length() == 1)
            return getString(digits.charAt(0));
        ArrayList<String> first = getString(digits.charAt(0));
        ArrayList<String> tmp = letterCombinations(digits.substring(1));
        
        for(int i = 0; i < first.size(); ++i)
        {
            for(int j = 0; j < tmp.size(); ++j)
            {
                result.add(first.get(i) + tmp.get(j));
            }
        }
        return result;
    }
    
    private ArrayList<String> getString(char mychar)
    {
        ArrayList<String> result = new ArrayList<String>();
        
        switch(mychar){
        case '2': result.add("a");
                  result.add("b");
                  result.add("c");
                  break;
        case '3': result.add("d");
                  result.add("e");
                  result.add("f");
                  break;
        case '4': result.add("g");
                  result.add("h");
                  result.add("i");
                  break;
        case '5': result.add("j");
                  result.add("k");
                  result.add("l");
                  break;
        case '6': result.add("m");
                  result.add("n");
                  result.add("o");
                  break;
        case '7': result.add("p");
                  result.add("q");
                  result.add("r");
                  result.add("s");
                  break;
        case '8': result.add("t");
                  result.add("u");
                  result.add("v");
                  break;
        case '9': result.add("w");
                  result.add("x");
                  result.add("y");
                  result.add("z");
                  break;
        default: break;
        }
        return result;
    }
}