Letter Combinations of a Phone Number

题目：

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"].

思路：

没啥思路，上来生做。。。就是permutation。一个bug就是，在check input之后，加一个空string到res里。

    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<String>();
        if (digits == null || digits.length() == 0) {
            return res;
        }
        //BUG: add empty string first! after dealing with the coner case.
        res.add("");

        int len = digits.length();
        HashMap<Character, String> key = new HashMap<Character, String>();
        key.put('1',"");
        key.put('2',"abc");
        key.put('3',"def");
        key.put('4',"ghi");
        key.put('5',"jkl");
        key.put('6',"mno");
        key.put('7',"pqrs");
        key.put('8',"tuv");
        key.put('9',"wxyz");
        
        for (int i = 0; i < len; i++) {
            char digit = digits.charAt(i);
            String letter = key.get(digit);
            ArrayList<String> newRes = new ArrayList<String>();
            for (int j = 0; j < letter.length(); j++) {
                for (String t : res) {
                    String subRes = t + letter.charAt(j);
                    newRes.add(subRes);
                }
            }
            res = newRes;
        }
        return res;
    }