Letter Combinations of a Phone Number

Q:
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.

T:
Use a vector to store previously generated combinations. For a new digit, append each of its characters to each of the previous combination.

A:

class Solution {
public:
    vector< string > letterCombinations(string digits) {
        string charmap[10] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector< string > ret;
        ret.push_back("");
        for(int i=0; i < digits.size(); i++) {
            string str = charmap[digits[i] - '0'];
            vector< string > tmp;
            for(int j=0; j < str.size(); j++) {
                for(int k=0; k < ret.size(); k++) {
                    tmp.push_back(ret[k]+str[j]);
                }
            }
            ret = tmp;
        }
        return ret;
    }
};