Leetcode Letter Combinations of a Phone Number

const char* lookup[] = {" ", "", "abc", "def",
                                "ghi", "jkl", "mno", 
                                "pqrs", "tuv", "wxyz" 
                        };
class Solution {
public:
    vector<string> letterCombinations(string digits) {   
        string str;
        vector<string> res;

        dfs(digits, 0, str, res);
        return res;
    }
    
    void dfs(string& digits, int pos, string& str, vector<string>& res) {
        if (digits.length() <=  pos) {
            res.push_back(str);
            return;
        }
        
        const char* alphabetas = lookup[digits[pos] - '0'];
        for (int i=0; alphabetas[i] != ''; i++) {
            str.push_back(alphabetas[i]);
            dfs(digits, pos + 1, str, res);
            str.pop_back();
        }
    }

};

dfs