Letter Combinations of a Phone Number

 1     public List<String> letterCombinations(String digits) {
 2         List<String> res = new ArrayList<String>();
 3         if (digits == null || digits.length() == 0) {
 4             return res;
 5         }
 6         //BUG: add empty string first! after dealing with the coner case.
 7         res.add("");
 8 
 9         int len = digits.length();
10         HashMap<Character, String> key = new HashMap<Character, String>();
11         key.put(‘1‘,"");
12         key.put(‘2‘,"abc");
13         key.put(‘3‘,"def");
14         key.put(‘4‘,"ghi");
15         key.put(‘5‘,"jkl");
16         key.put(‘6‘,"mno");
17         key.put(‘7‘,"pqrs");
18         key.put(‘8‘,"tuv");
19         key.put(‘9‘,"wxyz");
20         
21         for (int i = 0; i < len; i++) {
22             char digit = digits.charAt(i);
23             String letter = key.get(digit);
24             ArrayList<String> newRes = new ArrayList<String>();
25             for (int j = 0; j < letter.length(); j++) {
26                 for (String t : res) {
27                     String subRes = t + letter.charAt(j);
28                     newRes.add(subRes);
29                 }
30             }
31             res = newRes;
32         }
33         return res;
34     }