17. Letter Combinations of a Phone Number

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> output;
        if(digits.empty())
            return output;
        string dict[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        combination(digits,dict,0,"",output);
        return output;
    }
    void combination(string digits,string dict[],int index,string input,vector<string> &output){
        if(index == digits.size()){
            output.push_back(input);
            return;
        }
        string str = dict[digits[index] - ‘0‘];
        for(int i = 0;i < str.size();i++)
            combination(digits,dict,index+1,input + str[i],output);
    }
};