简单的回溯法！

class Solution {public:    void backTrack(string digits, vector
     
       words, string ans, vector
      
       & res, int k, int flag[])    {        if(k == digits.size())        {            res.push_back(ans);        }        else        {            for(int i=0; i
       
         letterCombinations(string digits) {        string ans;        int flag[4] = {
   0,};//0为未用过        vector
        
          words = {
   "","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        vector
         
           res;        if(digits == "")            return res;        backTrack(digits,words,ans,res,0,flag);        return res;    }};