主要实现任意长度大整数的四则运算，包括加减乘除，求余运算只需对除法稍做改进就行。因为这是帮朋友写的，所以是按他的要求用string实现的。如果有需要，也可以改为用定长数组或者vector来实现。-This document is mainly aimed to realize the operations of addition, substraction, multiplication and division of integers which are of any length.