We will hold AAL Contest 001: Let's use segtree!, an official educational contest focused on segment trees, on AtCoder.

• Contest URL: https://atcoder.jp/contests/aalc001
• Start Time: https://www.timeanddate.com/worldclock/fixedtime.html?iso=20261012T1300&p1=248
• Duration: 300 minutes
• Number of Problems: 24
• Problem statements will be provided in both English and Japanese.
• This contest is unrated. There will be no rating updates.
• Writers: Nyaan, maspy
• Tester: Nachia

What Is the AAL Contest?

AtCoder offers AtCoder Algorithm Lectures, a series of courses explaining various theories and techniques used in competitive programming. Currently, the lectures are available only in Japanese, but we plan to offer an English version in the future.
In addition to courses in the form of explanatory articles and videos, AtCoder Algorithm Lectures will host AAL Contests as educational resources that allow participants to study specific topics in greater depth and gain practical experience.
We hope that you will use these contests to test your skills on the day of the contest and as learning materials afterward, together with the Algorithm Lectures courses.

This Contest's Topic

The topic of the first contest is segment trees.
Segment trees are known as one of the most frequently used data structures among those that are not built into programming languages. They can process aggregate queries in logarithmic time, and by carefully choosing the monoid they store, they can maintain many different kinds of information.
This contest features problems involving many different kinds of information stored in segment trees. Those who are not yet familiar with segment trees are encouraged to work through the problems and learn the techniques for designing the information maintained by a segment tree.
In addition, every problem in this contest can be solved using the segment tree or lazy segment tree provided by AtCoder Library. In other words, extended segment-tree data structures, such as persistent segment trees and Segment Tree Beats!, are outside the scope of this contest. Those who do not have their own segment tree library are encouraged to use the official library.
The problems from Problem R onward require not only knowledge but also substantial problem-solving insight. Advanced participants are also encouraged to take on the contest!

We are looking forward to your participation!

I am one of the part-time staff members at AtCoder responsible for investigating cheating involving AI. Let me explain what happened during my free time today.

What I did in my free time today

I was browsing Codeforces blogs, as I often do for fun.

I found a post asking AtCoder to unban a certain user. The banned user was also in the comments, strongly insisting that they had not cheated. The comment section was full of people criticizing AtCoder. I immediately reviewed the case again. The user had been banned yesterday for cheating that took place three months earlier. After carefully rechecking the evidence, I also concluded that they had clearly used AI. (I will not explain the details because they relate to our methods for detecting AI use. The same applies to the cases below.)

The post also named another person who was said to have been falsely banned. I did not feel like spending even more time on this, but I carefully reviewed that case as well. That user had blatantly cheated one month earlier.

Next, I found another post in which someone claimed that AtCoder had banned them by mistake. I reviewed the case, and once again, the user had cheated.

A little later, another "wrongly banned user" was added to the first post. I reviewed that case as well. I doubt I need to tell you the result.

Finally, I found a comment on an AtCoder-related blog saying, "AtCoder banned me. I didn't cheat. Please unban me." I reviewed the case, and of course, that user had cheated too.

(Total time spent: a full two hours.)

Are you kidding me? Do all these people cheat and then somehow forget that they did it?! And the commenters kept making fun of AtCoder's banning system, without even knowing that we seriously review every claim of a false ban whenever we come across one!

Until now, I had taken claims of false bans on Codeforces seriously. However, after seeing so many claims from liars or people who had somehow forgotten their own cheating in a single day, I changed my mind. From now on, when I see a Codeforces post about a false ban, I will handle it as follows:

• If the author had a max rating of LGM as of the end of 2025, I will treat it as a serious appeal that could affect their entire competitive programming career, and I will carefully review the case.
• Otherwise, I will ignore it.

Unfortunately, this seems to be the most reasonable way to avoid being misled by cheaters' lies.

Please do not let cheaters convince you that AtCoder often bans innocent users by mistake. Also, if you find a blog post about a false ban by AtCoder, feel free to reply with a link to this article. Unfortunately, most people who write such posts are liars, people who have forgotten their own cheating, or fools who were tricked by cheaters into writing the post.

Some tips

• If you believe AtCoder banned you by mistake and want the ban removed, the right thing to do is contact AtCoder through its official support channel, rather than write a post on Codeforces. Also, AtCoder’s official ban policy is described in the following article: https://codeforces.me/blog/entry/154651
• This should be obvious, but just to be clear: even if you have entered 100 contests and competed honestly in 99 of them, cheating in even one makes you a cheater. And of course, AtCoder may ban you for cheating in a past contest.

We will hold Next DP Contest on AtCoder.

• Contest URL: https://atcoder.jp/contests/ndpc
• Start Time: https://www.timeanddate.com/worldclock/fixedtime.html?iso=20260505T1300&p1=248
• Duration: 300 minutes
• Number of Problems: 20 problems

This contest is a collection of practice problems on dynamic programming (DP). DP is one of the most important algorithms in competitive programming because of its high versatility and wide variety of applications. Therefore, studying DP is essential for gaining a deeper understanding of competitive programming. In the past, contests such as TDPC(Japanese language only) and EDPC were held for the purpose of practicing DP.

On the other hand, as time goes by, techniques become more diverse and more widely used. Now, 12 years after TDPC and 7 years after EDPC, DP techniques that did not appear in those contests have started to show up in contests, especially in ABC and ARC. This contest focuses mainly on techniques that were not covered in TDPC or EDPC. We hope you will use it both to test your skills during the contest and to review afterward.

We are looking forward to your participation!

Contest Page

Since there were more overseas participants than expected, I set up a discussion blog just in case.