Leetcode 17. Letter Combinations of a Phone Number

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        int n = digits.length();
        if(n == 0) {
            return result;
        }
        map<char, string> m = { 
            {'2', "abc"}, 
            {'3', "def"}, 
            {'4', "ghi"}, 
            {'5', "jkl"}, 
            {'6', "mno"}, 
            {'7', "pqrs"}, 
            {'8', "tuv"}, 
            {'9', "wxyz"}
        };
        vector<string> values;
        for(char ch : digits) {
            values.push_back(m[ch]);
        }
        traverse(result, values, n, 0, "");
        return result;
    }

private:
    void traverse(vector<string>& result, vector<string>& values, int& n, int current, string s) {
        if(current == n) {
            result.push_back(s);
            return;
        }
        for(char ch : values[current]) {
            string temp = s;
            temp += ch;
            traverse(result, values, n, current + 1, temp);
        }
    }
};