Letter Combinations of a Phone Number

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

class Solution {
public:
    vector<string> letterCombinations(string digits) 
    {
        vector<string> res;
        if(digits.length() == 0)
        {
            return res;
        }

        map<char, vector<string>> hash;
        hash['2'] = {"a", "b", "c"};
        hash['3'] = {"d", "e", "f"};
        hash['4'] = {"g", "h", "i"};
        hash['5'] = {"j", "k", "l"};
        hash['6'] = {"m", "n", "o"};
        hash['7'] = {"p", "q", "r", "s"};
        hash['8'] = {"t", "u", "v"};
        hash['9'] = {"w", "x", "y", "z"};

        string cur = "";
        dfs(res, cur, hash, digits, 0);
        return res;

    }

    void dfs(vector<string>& res, string& cur, map<char, vector<string>>& hash, string digits, int pos)
    {
        if(pos >= digits.length())
        {
            res.push_back(cur);
            return ;
        }

        for(string str : hash[digits[pos]])
        {
            cur.append(str);
            dfs(res, cur, hash, digits, pos+1);
            cur.pop_back();
        }

    }
};