回溯法: class Solution { public: //1. 用map记录每个数字按键对应的所有字母 map<char, string> M = { {'2', "abc"}, {'3', "def"}, {'4', "ghi"}, {'5', "jkl"}, {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"...