private static void Main(string[] args)
        {
            Student s3 = new Student("王五", null);

            Student s1 = new Student("张三", s3);
            Student s2 = new Student("李四", s1);

            Book b1 = new Book("赵六");
            s2.CheckBook(b1);

            Console.ReadLine();
        }
 public Student(string name, Student nextStudent)
 {
     _name = name;
     _nextStudent = nextStudent;
 }