Letter Combinations of a Phone Number

class Solution {public:    string dic[10] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};    vector<string> res;    void dfs(string& digits, string s, int cur, int len){        if(cur==len){            res.push_back(s);            return;        }        int c = digits[cur] - '0';        for(int i = 0; i < dic[c].size(); i++){            s.push_back(dic[c][i]);            dfs(digits, s, cur + 1, len);            s.pop_back();        }        return;    }    vector<string> letterCombinations(string digits) {        const int len = digits.length();        if(len == 0) return res;        string s;        dfs(digits, s, 0, len);        return res;    }};