Hello, Codeforces!

Artyom123, Kotehok3 and I are happy to invite you to Codeforces Round #671 (Div. 2), which will be held on Sep/19/2020 17:35 (Moscow time). This round will be rated for all participants, whose rating is lower than 2100 (We really hope so).

We would like to thank:

• isaf27 for coordinating the round and helping us <3
• dorijanlendvaj, physics0523, kdjonty31, -____-, postscript, Osama_Alkhodairy, Mohamed.Sobhy, morzer, saurabhshadow, thenymphsofdelphi, MagentaCobra, PM11, MarwanNabil, hugopm, Kirill22 for testing the round and giving us useful feedback
• MikeMirzayanov for great Polygon and Codeforces platforms

You will be given 6 problems and 2 hours to solve them (or to play Valorant, because the contest is related to it for some reason). One problem has an easy version and a hard version.

You will have to help the agents from the game to grind that rating. Try your best as if you are playing ranked and they are your teammates!

gl hf

Here is the score distribution:

500 — 750 — 1250 — (750 + 1000) — 2250 — 3000

“Codeforces… assemble!”

We are thrilled to announce Grakn Forces 2020; hosted by Codeforces and sponsored by Grakn Labs. Thank you to Codeforces for providing a world-class competition structure to partner with. We can’t wait to see what you all are capable of. This international competition consists of 1 round, lasting 2.5 hours. The contest will comprise 7-9 problems and will be in accordance with classic Codeforces rules.

Prizes:

• 1st place = 500 euros
• 2nd place = 250 euros
• 3rd place = 100 euros

Additional Prizes:

Top 50 receive:

Grakn Labs swag pack:

• Stickers
• Grakn Labs t-shirt

Random 50 from 51-250 receive:

Grakn Labs swag pack:

• Stickers
• Grakn Labs t-shirt

About Grakn Labs

We are the inventors of the Grakn knowledge graph and the Graql query language. Our technology helps organizations in various industries, including Life Sciences, Defence & Security, Financial Services, and Robotics, to build intelligent systems that will change the world. From financial analytics to drug discovery, cyber threat detection to robotics disaster recovery, our technology empowers engineers around the world to tackle a higher order of complexity in knowledge and solve the world's most complex problems.

Based in London, UK, and founded in 2016, Grakn Labs is a team of people driven by a purpose: to solve the world's most complex problems, through knowledge engineering. In 2017, we were awarded Product of the Year by the University of Cambridge. After only a few years, our pioneering community has grown to 6K+ engineers, spread across 10+ countries around the globe -- and we're still growing. To continue pursuing this vision, we need to build a team of individuals who are not just the best and brightest in what they do, but also driven by a strong sense of purpose and compassion for the world.

What is Grakn and Graql?

Grakn is a distributed knowledge graph: a logical database to organize large and complex networks of data as one body of knowledge. Grakn provides knowledge engineering tools for developers to easily leverage the power of Knowledge Representation and Reasoning when building complex systems. Ultimately, Grakn serves as the knowledge-base foundation for cognitive and intelligent systems.

Graql is Grakn's query language. It provides an expressive knowledge schema language through an enhanced entity-relationship model, transactional queries that perform deductive reasoning in real-time, and analytical queries with native distributed Pregel and MapReduce algorithms. Graql provides a strong abstraction over low-level data constructs and complex relationships.

Graql is distributed as an open-source technology, while Grakn comes in two forms: Grakn Core — which is also open-source, and Grakn KGMS — our enterprise Knowledge Graph Management Service, available through Google Cloud and Amazon AWS.

Hello Codeforces!

On Sep/14/2020 17:35 (Moscow time) Educational Codeforces Round 95 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Ne0n25 Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Congratulations to the winners:

And finally people who were the first to solve each problem:

UPD: Due to the issues with problems A and B the round is unrated.

UPD: Editorial is out

Hello Codeforces!

gyh20 and I are glad to invite you to Codeforces Round #670 (Div. 2) which will start on Sep/12/2020 16:45 (Moscow time). Note the unusual start time of the round.

The contest will last for two hours, and you will have five tasks to solve. The tasks are prepared by me and gyh20. This round is rated for participants whose rating is not higher than 2099. You can see that my current rating is exactly 2099 :)

There might be an interactive problem. You can learn about them here.

We would like to thank:

• isaf27 for coordinating and helping us with the problems.
• namanbansal013, postscript, morzer, Osama_Alkhodairy, csani, growup974, physics0523, bizhouziwei20 for testing the problems and providing useful feedback. Special thanks to csani for helping us with writing the editorials, and physics0523 for improving one problem's solution!
• c2020hxw, fisherking, fangzhijian2020 for discussing the problems with us.
• finally, MikeMirzayanov for great platforms Codeforces and Polygon!

We tried our best to make the statements short and clear, pretests strong and problems interesting. We hope you like the problems!

Score distribution will be announced shortly before the round.

Good luck and have fun!

Upd: Score distribution is 500-750-1250-1750-2500.

Upd: For problem reasons, the contest is delayed for 10 minutes. We are very sorry to keep you waiting, sorry again.

Upd: Score distribution is changed to 500-1000-1500-2000-2750.

Upd: The round is finished. We're really sorry for B being well-known (none of the testers knew the harder version of this problem in ABC173E). Still, congrarulations to the winners!

Div1 (unofficial):

1. WZYYN
2. Geothermal
3. kort0n
4. neal
5. kotatsugame

Div2:

1. JSoap
2. DemolitionLovers
3. killyou
4. gmh77
5. WaterCrysital

Upd: Editorial is out here.

Hello Codeforces!

My name is Vitaly Aksenov and I present you the new lesson in English EDU section as a part of "ITMO Academy: pilot course". I will talk about Disjoint Sets Union (DSU).

A little bit about myself, I cannot brag about being red (International Grandmaster), however, I was twice and each time afterwards there was a revolution of colours on Codeforces. :-) Right now I am the Researcher in ITMO University in parallel and distributed computing (link). Also, I am in Jury Committee of several olympiads such as NERC, Bioinformatics Contest, Russian Code Cup and etc.

This is my first time to write a lecture so do not judge harshly. I hope you will like it!

I want to thank pashka for video editing, and thanks to pashka, MikeMirzayanov and niyaznigmatul for sharing the problems.

More about EDU section you can read in this post.

realhype and I are glad to invite you to Codeforces Round #669 (Div. 2), which will take place on Sep/08/2020 17:35 (Moscow time). The round will be rated for all the participants with rating strictly less than 2100.

The problems were created and prepared by realhype and i.e. We would also like to thank everyone who was involved in the round preparation:

• antontrygubO_o for round coordination and high sense of taste that made our contest IDEAl.

• xiryss, Tlatoani, hugopm, Alireza_Keshavarz, jouIej, jo_ulej, dick, Ari, AmShZ, AliceWalkingOutOfHerLife, Devil, morzer, baby_baby_baby_ooh, PM11, postscript, Programmer, saurabhshadow, _overrated_, LordVoldebug, LordVoIdebug, DrNykterstein for high-quality testing.

• MikeMirzayanov for Polygon and Codeforces platforms.

You will have 2 hours to solve 5 tasks. One of them is interactive, you should know how to deal with them.

Special thanks to my amazing minipig Schweine, who inspired us during the entire preparation time.

Scoring will be announced shortly before the round.

UPD1: There will be no pictures of minipig in the statements. :)

UPD2: The scoring is 500-1000-1500-2000-2500.

UPD3: EdItOrIaL.

UPD4: Congrats to winners!

Div1 + Div2:

1. ksun48

2. WZYYN

3. I_love_chickpea

4. neal

5. jiangly

Div2:

1. 9623

2. deep_fake

3. GiannisAttemptafreethrow

4. watemus

5. chad_master_13221

Hello Codeforces!

We are glad to invite you to Codeforces Round #668 (Div. 1) and Codeforces Round #668 (Div. 2), which will take place on Sep/06/2020 17:35 (Moscow time).

The problems were created by Ari, Kuroni, Maripium, Monogon, antontrygubO_o and hugopm.

We would like to thank:

• dorijanlendvaj, Jatana and MrDecomposition for helping with the preparation of the tasks.

• gamegame for always being nice and supportive.

• DatVu, Maripium, MofK and atoiz for stealing the round #666, forcing us to retheme around ponies.

• Monogon for making the author list more colorful.

• TwentyFour, thenymphsofdelphi, qlf9, mcfr, Tlatoani, i_love_Lisa, McDic, cup_of_tea, yann-dev, Noam527, Mbfibat, wolfris, ffao, Falseeee, fpc_coder and fivefourthreeone for testing.

• MikeMirzayanov for the Codeforces and Polygon platforms.

There will be 5 problems in each division and will be given 2 hours to solve them (or post memes in the comments if you can't solve any).

We tried to make interesting problems, short statements, useful samples and strong pretests, and we hope you will like it.

Scoring will be announced shortly before the round and editorial will be posted shortly after the round (it is already written).

Good luck!

UPD: The number of problems is now 5 in each division.

UPD2: Score distribution:

• Div.1: 500 — 1000 — 1500 — 2250 — 3000
• Div.2 : 500 — 1000 — 1750 — 2250 — 3000

UPD3: Editorial is out

UPD4: We are sorry for problem div1E being well-known. No setter or tester have seen this problem. We apologize and hope you still enjoyed the contest.

UPD5: Congratulations to the winners:

Div.1

1. maroonrk
2. ainta
3. ko_osaga
4. Itst
5. hos.lyric

Div.2

1. minhchauxinhdep1234
2. potato167
3. guangmutianwang
4. w_o_kje
5. veckoper

Hello! Codeforces Round #667 (Div. 3) will start at Sep/04/2020 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of my work. Thanks to my good friends Daria ZeroAmbition Stepanova, Mikhail pikmike Piklyaev, Maksim Ne0n25 Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round. Also thanks to Artem Rox Plotkin and Dmitrii _overrated_ Umnov for the discussion of ideas and testing the round!

Good luck!

UPD: Huge thanks to Ivan Gassa Kazmenko for testing the round and fixing some issues with statements and the round in general! Also thanks to sladkayaKlubnichka, kocko, Ilya-bar and infinitepro for testing the round!

UPD2: Editorial is published!

Hello Codeforces!

I teach a course on algorithms and data structures at ITMO University. During the last year I was streaming all my lectures on Twitch and uploaded the videos on Youtube.

This year I want to try to do it in English.

This is a four-semester course. The rough plan for the first semester:

• Algorithms, complexity, asymptotics
• Sorting algorithms
• Binary heap
• Binary search
• Linked lists, Stack, Queue
• Amortized analysis
• Fibonacci Heap
• Disjoint Set Union
• Dynamic Programming
• Hash Tables

The lectures are open for everybody. If you want to attend, please fill out this form to help me pick the optimal day and time.

See you!