17. Letter Combinations of a Phone Number

class Solution {
public:
    vector<string>v={"","","abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    vector<string>result;
    string local;
    vector<string> letterCombinations(string digits) {
        if(digits.size()==0) return vector<string>();
        f(local,0,digits);
        return result;
    }
    void f(string &local,int index,string& digits)
    {
        
        if(index==digits.size())
            result.push_back(local);
        else
            for(int i=0;i<v[digits[index]-'0'].size();i++)
            {
                local.push_back(v[digits[index]-'0'][i]);
                f(local,index+1,digits);
                local.pop_back();
            }
    }
};