LeetCode Solutions : Letter combinations of a phone number

public class Solution {
    Map<Integer,List<String>> map;
    public List<String> letterCombinations(String digits) {
        map = new HashMap<Integer,List<String>>();
        this.map.put(2,new ArrayList<String>(Arrays.asList("a","b","c")));
        this.map.put(3,new ArrayList<String>(Arrays.asList("d","e","f")));
        this.map.put(4,new ArrayList<String>(Arrays.asList("g","h","i")));
        this.map.put(5,new ArrayList<String>(Arrays.asList("j","k","l")));
        this.map.put(6,new ArrayList<String>(Arrays.asList("m","n","o")));
        this.map.put(7,new ArrayList<String>(Arrays.asList("p","q","r","s")));
        this.map.put(8,new ArrayList<String>(Arrays.asList("t","u","v")));
        this.map.put(9,new ArrayList<String>(Arrays.asList("w","x","y","z")));
        return comb(digits);
       
    }
    public List<String> comb(String digits) {
        List<String> result = new LinkedList<String>();
        if(digits.length() == 0) {
            result.add(new String(""));
        } else if(digits.length() == 1) {
            List<String> ret = new LinkedList<String>();
            for(String c:map.get(Integer.parseInt(digits)))
                ret.add(c);
            return ret;
        } else {
            for(String s : comb(digits.substring(1))) {
                for(String c:map.get(Integer.parseInt(digits.substring(0,1))))
                    result.add(c + s);
            }
        }
        return result;
    }
}