[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.

下面是我AC的代码：

/**
 * author:Zhou J
 */

class Solution 
{
public:
    vector<string> letterCombinations(string digits) 
    {
        map<char, string> digitToString;
        digitToString.insert(make_pair('0', ""));
        digitToString.insert(make_pair('1', ""));
        digitToString.insert(make_pair('2', "abc"));
        digitToString.insert(make_pair('3', "def"));
        digitToString.insert(make_pair('4', "ghi"));
        digitToString.insert(make_pair('5', "jkl"));
        digitToString.insert(make_pair('6', "mno"));
        digitToString.insert(make_pair('7', "pqrs"));
        digitToString.insert(make_pair('8', "tuv"));
        digitToString.insert(make_pair('9', "wxyz"));
        
        vector<string> ret;
        string path;
        letterCombinationsRec(digits, ret, path, digitToString);
        return ret;
    }
    
    void letterCombinationsRec(const string &digits,
                               vector<string> &ret,
                               string &path,
                               map<char, string> &digitToString)
    {
        if (path.size() == digits.size())
        {
            ret.push_back(path);
            return;
        }
        
        for(const auto &digit :  digitToString[digits[path.size()]])
        {
            path.push_back(digit);
            letterCombinationsRec(digits, ret, path, digitToString);
            path.erase(path.size() - 1);
        }
    }
};