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"].

Solution:

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