Letter combination of a phone number -- Leetcode

Question:

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"].

Answer:
C++ version:
class Solution {
public:
    vector<string> letterCombinations(string digits) {
        string mmap[] = {"", " ", "abc", "def", "ghi", "jkl",
        "mno", "pqrs", "tuv", "wxyz"};
        vector<string> res;    
        string seq;
        Generater(mmap, digits, 0, seq, res);
        return res;
   }
   void Generater(string mmap[], string& digits, int level, string& subres, vector<string>& res){
       if(level == digits.size()){
           res.push_back(subres);
           return;
       }
       int digint = digits[level] - '0';
       for(int i =0; i < mmap[digint].size(); i++){
           subres.push_back(mmap[digint][i]);
           Generater(mmap, digits, level+1, subres,res);
           subres.resize(subres.size() -1);
      }
   }
};



Java version:
public class Solution {
   
    public List<String> letterCombinations(String digits) {
        String[] mmap = {""," ","abc","def","ghi","jkl", "mno", "pqrs", "tuv", "wxyz"};
        List<String> res = new ArrayList<String>();
        String subres = new String();
        generate(mmap, digits, 0, subres, res);
        return res;
    }
   
    public void generate(String[] mmap, String digits, int idx, String subres, List<String> res){
        if(idx == digits.length()){
            res.add(subres);
            return;
        }
       
        int digit = digits.charAt(idx) - '0';
        for(int i=0; i<mmap[digit].length();++i){
            subres += mmap[digit].charAt(i);
            generate(mmap, digits, idx+1, subres, res);
            subres = subres.substring(0,subres.length()-1);
        }
        return;
    }
}