LeetCode 17 Letter Combinations of a Phone Number

class Solution
{
public:
    void init()
    {
        int count = 0;
        for(int i = 1; i<9; ++i)
        {
            string temp;
            for(int j=0; j<3; ++j)
            {
                temp.push_back(‘a‘+ count);
                count++;
            }
            if(i==6 || i==8)
            {
                temp.push_back(‘a‘+count);
                count++;
            }
            album.push_back(temp);
        }
    }
    void getcombination(string &digits, int i, int n, string str)
    {
        if(i == n)
        {
            ret.push_back(str);
            str.clear();
            return;
        }
        for(int j=0; j != album[digits[i]-‘2‘].size(); ++j)
        {
            getcombination(digits, i+1, n, str + album[digits[i] -‘2‘][j]);
        }
    }
    vector<string> letterCombinations(string digits)
    {
        if(digits.empty())
        {
            return ret;
        }
        string str;
        init();
        getcombination(digits, 0, digits.size(), str);
        return ret;
    }
    
private:
    vector<string>  album;
    vector<string>  ret;
};