leetcode[17]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:

void createMap(map<char,string> &dMap)

{

    dMap['0']="";

    dMap['1']="";

    dMap['2']="abc";

    dMap['3']="def";

    dMap['4']="ghi";

    dMap['5']="jkl";

    dMap['6']="mno";

    dMap['7']="pqrs";

    dMap['8']="tuv";

    dMap['9']="wxyz";

    return;

}

void dfsDigits(int curDeep, vector<string> &res, string currStr, const string digits, map<char,string> dMap)

{

    if (curDeep==digits.size())

    {

        res.push_back(currStr);

        return;

    }    

    for (int i=0; i<dMap[digits[curDeep]].size();i++)

    {

        dfsDigits(curDeep+1, res, currStr+dMap[digits[curDeep]][i], digits, dMap);

    }

}

vector<string> letterCombinations(string digits) 

{

    vector<string> res;

    const int len=digits.size();

    map<char,string> dMap;

    createMap(dMap);

    dfsDigits(0, res, "", digits, dMap);

    return res;

}

};