leetcode letter combinations of a phone number

class  {
private:
    vector<string> mapping = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits.size() == 0) return res;
        int index = 0;
        string value = "";
        dfs(index, digits, res, value);
        return res;
    }
    void dfs(int index, string digits, vector<string>& res, string& value) {
        if(index == digits.size()) {
            res.push_back(value);
            return;
        }
        for(int i = 0; i < mapping[digits[index] - '2'].size(); ++i) {
            value.push_back(mapping[digits[index] - '2'][i]);
            dfs(index + 1, digits, res, value);
            value.pop_back();
        }
    }
};