Letter Combinations of a Phone Number [Leetcode]

Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note: Although the above answer is in lexicographical order, your answer could be in any order you want.

Solution: nothing but DFS (After finishing this one, you may feel better to solve problem Word Break II).

class Solution {
public:
    void generator(string digits, vector<string>& ret, string s){
        string dict[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        int n = digits.size();
        if(n==0){
            ret.push_back(s);
            return;
        }
        int curDigit = digits[0]-'0';
        for(int i=0; i<dict[curDigit].size(); ++i){
            generator(digits.substr(1, n-1), ret, s+dict[curDigit][i]);
        }
    }
    vector<string> letterCombinations(string digits) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<string> ret;
        generator(digits, ret, "");
        return ret;
    }
};