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"].

public class Solution {    public List
     
       letterCombinations(String digits) {        List
      
        result=new ArrayList
       
        ();        if(digits==null || digits.length()==0){            return result;        }        List
        
          list=new ArrayList
         
          ();        for(int i=0; i
          
            inputs, int step,StringBuilder sb,List
           
             res){ if(step==inputs.size()){ res.add(sb.toString()); return; } String str=inputs.get(step); for(int i=0; i
           
          
         
        
       
      
     

　　二刷，也可以用BFS来写：

class Solution {    public List
     
       letterCombinations(String digits) {        List
      
        res = new ArrayList<>();        if(digits == null || digits.length() == 0){            return res;        }        String[] strArr = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        //int len = digits.length();        for(char c : digits.toCharArray()){            List
       
         temp = new ArrayList<>();            for(char ac : strArr[c - '0'].toCharArray()){                if(res.isEmpty()){                    temp.add(""+ac);                }                else{                    for(String str : res){                        StringBuilder sb = new StringBuilder(str);                        sb.append(ac);                        temp.add(sb.toString());                    }                }            }            res = temp;        }        return res;            }    }