【LeetCode】17. Letter Combinations of a Phone Number

题目：

思路：设置两个List,一个存储当前层，一个存储最终层

public class Solution {
    public List<String> letterCombinations(String digits) {
       List<String> listRet=new ArrayList<String>();
       if(digits==""){
           return listRet;
       }
       Map<Character,String> map=new HashMap<Character,String>();
       map.put('0',"0");
       map.put('1',"1");
       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");
       
       for(int i=0;i<digits.length();i++){
           List<String> listTem=new ArrayList<String>();
           if(i==0){
               char[] initials=map.get(digits.charAt(0)).toCharArray();
               for(char initial:initials){
                   listRet.add(String.valueOf(initial));
               }
               continue;
           }
           String s1=map.get(digits.charAt(i));
           char[] letters=s1.toCharArray();//当前数字对应字符串
           for(char letter:letters){
              for(String pre:listRet){
                  listTem.add(pre+String.valueOf(letter));//前缀+当前层字符
              }
           }
           listRet=listTem;
       }
       return listRet;
    }
}