The origin of the problem, either on BOJ or Yandex was given out during the contest but one can't simply copy the solution from those OJs(You'll need an account on Yandex, and you can view the submissions only when having solved it on BOJ).

Where Chinese contestants got the solution from was here.

What makes things more interesting is that the author of the blog is the rank 10 of Div.1 and if you check the publishing date, you can find that the blog is written/edited(more likely the latter) during the contest at 22:52(ATC+8).

Just feeling curious.

UPD: He deleted the blog. To be brief, it's the editorial of the ACM-ICPC round where the original problem appeared.

UPD2: Alternative url: https://web.archive.org/web/20220724163250/https://www.cnblogs.com/Flying2018/p/acmicpc2874.html