I've participated in JOI 2017 and I'm trying to find the solutions in English. Why don't we create a tutorial with brief explainings of the solutions? I'll contribute by posting the two problems I have solved during the contest, but I'll update when people comment solutions.
If someone speaks Japanese, the official JOI website has posted solutions, but they're only available in Japanese, so you could translate a brief explanation to us :)
Links for problem statements (English) are available below.