对一些超出整型数范围的正整数的应用，需要自己想办法解决的存储和运算。一种解决方案是用单链表存储一个正整数。注意，数字是用逆序存储的。试设计一个类处理任意大整数，必须实现输入输出操作、加法操作和赋值操作。(For some applications that are beyond the integer range of positive integers, they need to store and compute their own solutions. One solution is to store a positive integer with a single linked list. Notice that the numbers are stored in reverse order. Try to design a class to handle any large integer. You must implement input and output operations, addition operations, and assignment operations.)