class Solution {public:    string mapping[8]={"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};    void get_string(string &digits,int l,vector
    
      &ans,string &x){        if(l==digits.size()){            ans.push_back(x);            return;        }        for(auto &e:mapping[digits[l]-'2']){            x.push_back(e);            get_string(digits,l+1,ans,x);            x.pop_back();        }                    }    vector
     
       letterCombinations(string digits) {        /*        注意考虑digits空的情况。        */        vector
      
        ans;        if(digits.empty()) return ans;        string x;        get_string(digits,0,ans,x);        return ans;    }};