This is the first of a series of blog posts about the task `meetings' from IOI18. It is also available in a contest that will take place in conjunction with these posts.

We chose this problem as an experiment to see whether we can present a single problem in a way that it covers a lot of useful knowledge for a contest related topic. In this aspect, meetings is a great task in that it starts with dynamic programming, then moves into range queries and tree constructions, and finally, finishes it off with geometric data structures.

Furthermore, the setting of meetings is as classical as a data structure problem can get: you get a static array, and you query about some statistic on its subintervals. This is a setting that has been viewed as "completely understood" by most competitors who aim for top 40 or higher at the IOI since around 2005. However, this problem was given, and had 0 solvers. I've also heard several past contestants who placed in the Top-3 at IOIs refer to this problem as "Godly", so that's another reason to discuss it in more details.

In short, the ideas underlying meetings are extremely novel, especially given how well understood the topic is. Also, if mechanically you can solve meetings in 2 hours reliably, you shouldn't have trouble with most data structural topics.

Continue reading...

Recently, we have rolled out Two Factor Authentication! You can enable it on your edit profile page in order to make your account more secure.

For those that do not know how, the following are the steps that you can take:

1. Install Google Authenticator, Authy, or any other similar apps that provide the functionality.
2. Scan the QR Code with your device, or enter the code provided manually.
3. Enter the code shown by your device to sanity check and complete the enabling process.
4. Henceforth, your device should provide the 6-digit code that will be used for Two Factor Authentication.

Please note that the time on your device is required to be correct as the codes provided are time-based.

Recently, on the ACC1 (Another Contest), there was a fairly high profile attempt of utilizing fake accounts.

While such attempts may be done partly for humor, the nature of the DMOJ contests (with their long open durations) means such behavior could completely destroy the usefulness of the ranking system. We reached out to the likely users (according to xiaowuc1 it was not too hard to identify who they were by reading their code), and they agree with us about the seriousness of their actions. Please see below for their message.

In fact, we would like to use this opportunity to encourage users who would like to compete at national levels (or higher) to add their real-world information onto the about pages of their profiles. Yes, this adds additional stress for doing the contest (due to feeling like being watched), but it's also an essential part of the contest process.

Hello DMOJ,

We wanted to apologize for cheating on Another Contest 1 (10/12/2018). We took the contest as a team of 3 people with 3 computers on a fake account (y0105vv49). We understand that this is very unsportsmanlike, as it directly affects the rankings of real contestants taking the contest legitimately and sets a bad example for other users. Some of us have previously been reprimanded for this issue before, but we evidently failed to learn our lesson. We won't do this again. If there's any contributions to the DMOJ community we can make to make up for this, such as setting a contest, we'd be eager to volunteer our time.

Andrew He, Kevin Sun, Yinzhan Xu

Update: As of 12:07pm EDT today, the affected users and their data should be restored. If there is any problem, please don't hesitate to contact us on slack or at [email protected] Again, we apologize for the inconvenience.

At approximately 10:10am EST today (October 15th), we experienced a database error that led to the loss of several thousand user accounts, as well as related submissions. We are in the process of restoring from a backup from yesterday (October 14th), 8pm EST. Users whose accounts were affected will be rolled back to their backed up state, while users who are not should see no changes.

To prevent possible collisions while restoring, we have temporarily blocked logins and new registrations.

We are sorry for any inconvenience this may cause.

Thanks for participating in the DMOPC '18 October contest!

The winners are:

1. xiaowuc1 (perfect score)
2. d (perfect score)
3. whzzt (perfect score)
4. fjzzq2002 (perfect score)
5. yjq_naiive (perfect score)

Editorials for each of the problems have been uploaded to the respective problem pages.

We hope you enjoyed the contest, and see you next month!

On Tuesday, October 9th, we'll be hosting the October DMOJ Monthly Open Programming Contest!

Anyone with a DMOJ account is welcome to participate, and anyone without one is welcome to register and participate.

Contestants may participate in any 3-hour window between noon EDT of Oct. 9th and noon EDT of Oct. 10th. Please see the contest page for more details.

See you soon!

We're excited to invite you to Another Contest - a new contest series being hosted on the DMOJ platform!

The first contest will run from October 12th to October 16th. Hope to see you there!

Thanks for participating in the DMOPC '18 September contest!

The winners are:

1. xiaowuc1 (perfect score)
2. y0105w49 (perfect score)
3. Egor (perfect score)
4. zhouzixiang2004 (perfect score)
5. wleung_bvg

Editorials for each of the problems have been uploaded to the respective problem pages.

We hope you enjoyed the contest, and see you next month!

On Tuesday, September 11th, we'll be hosting the September DMOJ Monthly Open Programming Contest!

Anyone with a DMOJ account is welcome to participate, and anyone without one is welcome to register and participate.

Contestants may participate in any 3-hour window between noon EDT of Sept. 11th and noon EDT of Sept. 12th. Please see the contest page for more details.

See you soon!

It's that time of the (school) year again, and to celebrate, we present the Back to School '18 contest! Get back into your tip top programming shape and start the school year with a new rating!

Back to School '18 will consist of 8 problems, with difficulty ranging between CCC Junior to IOI level.

You can choose a single 4 hour period between September 7 and September 9 EDT.

See the contest page for more details.

The University of Toronto Schools is hosting the second UTS Open! The problem setters for this contest are PlasmaVortex, zxyl, Cthulhu, and insect.

This is a 4-hour online contest, which allows contestants to participate in any 4-hour window from August 22nd to August 24th. Please see the contest page for more details.

Triway Cup is a contest that will be hosted at Triway Education (9011 Leslie St Unit 306, Richmond Hill, ON L4B 3B6) on the 26th of August, at 10AM - 1PM! Registration starts at 9AM.

There will be 8 problems, ranging from 3 to 30 points, between CCC and CCO level. Teams of up to 2 are allowed.

Problem writers/testers of this round are imaxblue and AQT.

Thank you to aeternalis1 and kassuno for testing!

There will be free prizes, food, tons of fun and it's an excellent way to get to know the community! During the contest, snacks will be provided. Lunch and solutions happen immediately after the contest.

The prizes will be $75, $50, and $25 for first, second, and third place in the live event.

Also, fill out the information please! Triway Cup Registration

Note: Prizes are only available for Canadian high school students.