Leetcode 17. Letter Combinations of a Phone Number

题目：

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.

思路：

首先判断输入是否为空，如果是，则返回一个空的vector
构造一个map来存储数字与字符的对应关系
temppre存储每一个按键后新的结果，
依次对每一个按键进行遍历，得到最终结果

代码：

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        if(digits.empty()) return vector<string>();
        vector<string> res;
        string charmap[10] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.push_back("");
        for (int i = 0; i < digits.size(); i++)
        {
            vector<string> tempres;
            string chars = charmap[digits[i] - '0'];

            for (int c = 0; c < chars.size();c++)
                for (int j = 0; j < res.size();j++)
                    tempres.push_back(res[j]+chars[c]);
            res = tempres;
        }
        return res;
    }
};