class Solution {

     List<String> temp=new ArrayList<String>();

     Map<String,String> map=new HashMap<String,String>(){{

         put("2","abc");

         put("3","def");

         put("4","ghi");

         put("5","jkl");

         put("6","mno");

         put("7","pqrs");

         put("8","tuv");

         put("9","wxyz");

     }};

     public void back(String before,String next){

         if(next.length()==0){

             temp.add(before);

             return;

         }

         for(int i=0;i<(map.get(next.substring(0,1))).length();i++){

             back(before+(map.get(next.substring(0,1))).substring(i,i+1),next.substring(1));

         }

     }

     public List<String> letterCombinations(String digits) {

         if(digits==null||digits.length()<1){

             return temp;

         }

         back("",digits);

         return temp;

     }

 }