17 Letter Combinations of a Phone Number

public class Solution {
    Map<Character, String> map = new HashMap<Character, String>();
	List<String> res = new ArrayList<String>();
	public List<String> letterCombinations(String digits) {
		if(digits==null||digits.length()==0) return res;
        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");
        String curStr = new String();
		dfs(0, digits.length(), curStr, digits);
		return res;
    }
	
	public void dfs(int cur, int len, String curStr,String digits){
		if(cur==len){
			res.add(curStr);
			return;
		}else{
			char c = digits.charAt(cur);
			String s = map.get(c);
			int s_len = s.length();
			for(int i=0;i<s_len;++i){
				String tmp = new String(curStr+s.charAt(i));
				dfs(cur + 1, len, tmp, digits);
			}
		}
	}
}