Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

Analyse: Be careful about the difference between empty string and non-empty string when conducting an operation. If you set an empty string, you don't have an access to any index, the only operation you can do is contat/'+'. 

Runtime: 4ms

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> result;
        if(digits.empty()) return result;
        
        string temp(digits.size(), 0);
        helper(digits, result, temp, 0);
        return result;
    }
    
    void helper(string digits, vector<string>& result, string temp, int depth){
        if(depth == digits.size()){
            result.push_back(temp);
            return;
        }
        
        string info[] = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        string current = info[digits[depth] - '0']; //the size of the current string that the digit represent
        for(int i = 0; i < current.size(); i++){
            temp[depth] = current[i];
            helper(digits, result, temp, depth + 1);
        }
    }
};