说明：  实现一个字符串类myString。要求有构造函数，析构函数，拷贝构造函数，运算符 重载函数，可以实现串比较，串赋值，求串长，串连接，反转字符串等基本功能。
(Implement a string class myString. Require a constructor, destructor, copy constructor, operator overloading function can be achieved string comparison, string assignment, seeking a long string, string concatenation, reverse the basic functions of the string and so on.)