Letter Combinations of a Phone Number

class Solution {
public:
    vector<string> res;
    vector<string> letterCombinations(string digits) {
        string mapping[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.clear();
        string s;
        letterCombinationsRe(digits, mapping, s);
        return res;
    }
    void letterCombinationsRe(const string &digits, string mapping[], string &s)
    {
        if (s.size() == digits.size())
        {
            res.push_back(s);
            return;
        }
        string &letters = mapping[digits[s.size()]-'2'];
        for (int i = 0; i < letters.size(); ++i)
        {
            s.push_back(letters[i]);
            letterCombinationsRe(digits, mapping, s);
            s.pop_back();
        }
    }

};

class Solution {
public:
unordered_map<char, string> dig2char;
Solution()
{
    dig2char['2'] = "abc";
    dig2char['3'] = "def";
    dig2char['4'] = "ghi";
    dig2char['5'] = "jkl";
    dig2char['6'] = "mno";
    dig2char['7'] = "pqrs";
    dig2char['8'] = "tuv";
    dig2char['9'] = "wxyz"; 
}  
vector<string> letterCombinations(string digits) {
    vector<string> result;
    result.push_back("");
    if ( digits.size() == 0 )  
    {
        return result;
    }
    int n=digits.length();
    for(int i=0;i<n;i++){
        vector<string> tmp;
        for(int j=0;j<dig2char[digits[i]].length();j++){
            for(int k=0;k<result.size();k++){
                tmp.push_back(result[k]+dig2char[digits[i]][j]);
            }
        }
        result=tmp;
    }
  

    return result;
}

vector<string> letterCombinations2(string digits) {
    vector<string> result;
    if ( digits.size() == 0 )  
    {
        result.push_back("");
        return result;
    }      
    vector<string> temp = letterCombinations(digits.substr(1));
    for(int j = 0; j < dig2char[digits[0]].size(); j ++)
        for(int t = 0; t < temp.size(); t ++ )
            result.push_back(dig2char[digits[0]][j] + temp[t]);    

    return result;
}
};