17. Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

AC code:

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if (digits.length() == 0) return res;
        vector<string> num_map = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.push_back("");
        for (int i = 0; i < digits.length(); ++i) {
            vector<string> tem;
            string str = num_map[digits[i]-'0'];
            for (int j = 0; j < str.length(); ++j) {
                for (int k = 0; k < res.size(); ++k) {
                    tem.push_back(res[k] + str[j]);
                }
            }
            res = tem;
        }
        sort(res.begin(), res.end());
        return res;
    }
};

Runtime: 0 ms, faster than 100.00% of C++ online submissions for Letter Combinations of a Phone Number.