17. Letter Combinations of a Phone Number(C++)

Solution :

class Solution {
public:
    vector<string> letterCombinations(string digits) {
       if(digits.empty()) return vector<string>();
       vector<string> list={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
       vector<string> res;
       res.push_back("");
       for(int i=0;i<digits.size();i++){
           if(digits[i]>'9'||digits[i]<'0') continue;
           int num=digits[i]-'0';
           vector<string> tmp;
           for(int k=0;k<list[num].size();k++){
               for(int j=0;j<res.size();++j){
                   tmp.push_back(res[j]+list[num][k]);
               }
           }
           res.swap(tmp);
       }
       return res;
    }
};