leetcode 17 电话号码的数字组合

 1 class Solution {
 2     List<String> temp=new ArrayList<String>();
 3     Map<String,String> map=new HashMap<String,String>(){{
 4         put("2","abc");
 5         put("3","def");
 6         put("4","ghi");
 7         put("5","jkl");
 8         put("6","mno");
 9         put("7","pqrs");
10         put("8","tuv");
11         put("9","wxyz");
12         
13     }};
14     public void back(String before,String next){
15         if(next.length()==0){
16             temp.add(before);
17             return;
18         }
19         for(int i=0;i<(map.get(next.substring(0,1))).length();i++){
20             back(before+(map.get(next.substring(0,1))).substring(i,i+1),next.substring(1));
21         }
22     }
23     public List<String> letterCombinations(String digits) {
24         if(digits==null||digits.length()<1){
25             return temp;
26         }
27         back("",digits);
28         return temp;
29     }
30 }