https://blog.wongcw.com/2021/10/30/%e7%8f%be%e4%bb%a3-%e8%b5%b7%e4%ba%9e%e8%81%af%e5%90%88factorial%e9%96%8b%e7%99%bc%e5%9b%ba%e6%85%8b%e9%9b%bb%e6%b1%a0/