[LeetCode/LintCode] Letter Combinations of a Phone Number

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.

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