LeetCode 17. 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"].

answer:

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> letter = {"0","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        vector<string> result1,result2;
        result1.push_back("");
        for(int i = 0; i < digits.length(); i ++){
            result2.clear();
            vector<string>(result2).swap(result2);
            int index = digits[i] - '0';
            string temp = letter[index];
            for(int j = 0; j < temp.length(); j ++){
                for(int k = 0; k < result1.size(); k ++){
                    result2.push_back(result1[k] + temp[j]);
                }
            }
            result1 = result2;
        }
        return result2;
    }
};