Function factorial 

pub fn factorial(n: u32) -> Option<u64>

n! — faktorial (iterativ, overflowga xavfsiz).

u64 da 20! gacha sig’adi; kattasida None.

Misol

use rust_algorithms::numbers::factorial;

assert_eq!(factorial(5), Some(120));
assert_eq!(factorial(0), Some(1));
assert_eq!(factorial(21), None);