https://hellohasan.com/2016/11/23/%e0%a6%b0%e0%a6%bf%e0%a6%95%e0%a6%be%e0%a6%b0%e0%a7%8d%e0%a6%b8%e0%a6%bf%e0%a6%ad-factorial-recursive-function-recursion-2/