17 Letter Combinations of a Phone Number

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"].

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> dict = vector<string>({"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"});
        string comb;
        vector<string> res;
        if (digits.size()<=0)
            return res;
        help(digits,0,comb,res,dict);
        return res;
    }
    void help(string s, int i, string &comb, vector<string> &res, vector<string> &dict){
        if (i>=s.size()){
            res.push_back(comb);
            return;
        }
        string temp = dict[s[i]-'0'];
        if (temp.size()==0)
            help(s,i+1,comb,res,dict);
        else{
            for (int j=0;j<temp.size();j++){
                comb.push_back(temp[j]);
                help(s,i+1,comb,res,dict);
                comb.pop_back();
            }
        }
    }
};