17_Letter_Combinations_of_a_Phone_number

class Solution {
private:
    vector<string> dict = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    void helper(vector<string>& result, string current, int start, string digits){
        if (start == current.size()){
            result.push_back(current);
            return;
        }
        int num = digits[start] - '2';
        for (int i = 0; i < dict[num].size(); ++i){
            current[start] = dict[num][i];
            helper(result, current, start + 1, digits);
        }
        return;
    }
public:
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if (digits.empty()) return result;
        string current (digits.size(), 0);
        helper(result, current, 0, digits);
        return result;
    }
};