https://knightzone.studio/2008/11/16/921/%e5%b0%8f%e8%aa%aa%e5%89%b5%e4%bd%9c%ef%bc%9a%e5%ad%b8%e5%9c%92%e6%8e%a2%e9%9a%aa%e7%a4%be2-101-the-blocks-problem/