说明：  这是一个计算大数阶乘的算法． 主要是通过链表来保存结果的每一位数, 这样就不会有像long或int类型时,数太大而导致溢出
(This is an algorithm for calculating the factorial of large numbers. The main method is to store each digit of the result through a linked list, so that there is no overflow when the number is too large for a type like long or int)