[LeetCode] 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) {
        string path;
        vector<string> res;
        int step = 0;
        dfs(step, path, res, digits);
        return res;
    }
    void dfs(int step, string &path, vector<string> &res, const string &digits)
    {
        const static string strT[10] = {"", "", "abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        if(step == digits.size())
        {
            res.push_back(path);
            return;
        }
        for(int i = 0; i < strT[digits[step]-'0'].size(); i++)
        {
            path.push_back(strT[digits[step]-'0'][i]);
            dfs(step+1, path, res, digits);
            path.pop_back();
        }
    }
};