17.电话号码的字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

电话号码

示例：
输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

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class Solution {
public:
    vector<string> letterCombinations(string digits) {
        //题目说的2-9
        vector<string>s1, s2;
        map<char, vector<string>>mp;
        mp['2'] = { "a", "b", "c" };
        mp['3'] = { "d", "e", "f" };
        mp['4'] = { "g", "h", "i" };
        mp['5'] = { "j", "k", "l" };
        mp['6'] = { "m", "n", "o" };
        mp['7'] = { "p", "q", "r", "s" };
        mp['8'] = { "t", "u", "v" };
        mp['9'] = { "w", "x", "y", "z" };
        for (auto &ch : digits)
        {
            int size = s1.size();
            auto &v = mp[ch];
            for (auto &s : v)
            {
                for (int i = 0; i < size; i++)
                {
                    string temp = s1[i] + s;
                    s2.push_back(temp);
                }
                if (size == 0)s2.push_back(s);
            }
            swap(s1, s2);
            s2.clear();

        }
        return s1;

    }
};