E题"大风车"题解：
https://www.bilibili.com/video/BV164411k7e3
出自2011IMO:
https://artofproblemsolving.com/wiki/index.php?title=2011_IMO_Problems/Problem_2

之前出现过的另一场
https://ac.nowcoder.com/acm/contest/5891/E

也许是因为3blue1brown。
毕竟2020昆明站已经发生过一次了对不对。