【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"].
 * Note:
 * Although the above answer is in lexicographical order, your answer could be in any order you want.
 */
#include <iostream>
#include <string>
#include <vector>
#include <unordered_map>
using namespace std;

unordered_map<char, string> M = { 
    {'0', ""},
    {'1', ""},
    {'2', "abc"},
    {'3', "def"},
    {'4', "ghi"},
    {'5', "jkl"},
    {'6', "mno"},
    {'7', "pqrs"},
    {'8', "tuv"},
    {'9', "wxyz"} 
};

vector<string> letterCombinations(string digits) 
{
    string s1;
    vector<string> rs, tmp;

    rs.push_back("");
    for (int i = 0; i < digits.length(); i++)
    {
        s1 = M[digits[i]];  
        if (s1.empty()) continue;

        tmp.clear();
        for (int j = 0; j < s1.length(); j++)
        {
            for (int k = 0; k < rs.size(); k++)
            {
                tmp.push_back(rs[k] + s1[j]);
            }
        }
        rs = tmp;
    }
    return rs;
}

int main()
{
    vector<string> rs;

    rs = letterCombinations("234");
    for (int i = 0; i < rs.size(); i++)
    {
        cout << rs[i] << "  ";
    }
    cout << endl;

    system("pause");
    return 0;
}