leetcode 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"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

class Solution {
    static const char *letter[10];
    void Get_string(vector<string> &v,string &digit,int n,string str)
    {
        if (n >= digit.size()){
            v.push_back(str);
            return ;
        }
        
        string s = letter[digit[n] - '0'];
        for (int i =0;i <s.size();++i){
            Get_string(v,digit,n+1,str + s[i]);
        }
    }
public:
    vector<string> letterCombinations(string digits) {
        vector<string> v;
        if (digits.empty()) return v;
        string str;
        Get_string(v,digits,0,str);
        return v;
    }
};
const char *Solution::letter[10]= {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};