c# - Factorial Algorithm which uses no Big-Integer library -
i'm studing computer engineering , have hard task. we have develop c# application can calculate factorial of big numbers without bigint . mean big numbers, 123564891...82598413! . , don't have use permission use custom libraries (like bigint ) . researched , found few questions this, this question different others because have calculate really big numbers without custom libraries . found poormans algorithm . it's calculating 10000 . it's not enough us. with teammates, found solution . let's factorial of 123 . 123 string . , , sum 123 , 122 times (it's equal 123 x 122) . , sum result 121 times. goes until reach 1. so, sum 2 strings. we create algorithm summing strings. getting last char of first number (3 of 123) integer (we can use integer, not bigint) . , last char of second number integer (2 of 122). sum them , found result number's last char (result = x...x5) . last char first char. result number . know, should use while() or for() loop, ,...