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, congratulations 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!

Hello Codeforces!

We are teaming up with Polyup to offer a unique work-study apprenticeship for a Full-Stack Developer, right here in sunny Barcelona!

Based in Silicon Valley, this progressive EdTech company focuses on providing access to quality math and computational thinking programs at a global scale. Want to join this dynamic team of tech-disruptors? See the details below!

Apprenticeship Summary:

• Full tuition scholarship to study a Master’s degree in Computer Science for one year (scholarship value: €22,900)
• 3 hours of classes per day at Harbour.Space
• Internship at Polyup (remote)
• Competitive compensation
• Opportunity to join Polyup full-time after graduation

Please note preselected candidates will be requested to pay a non-refundable application fee of 125€ to study at Harbour.Space University.

Apply now at: https://www.linkedin.com/jobs/view/1998101008/

Looking forward to seeing your applications!

(Translation: Hello)

We are very happy to invite you to take part in our contest, Codeforces Round #666 (Div. 1) and Codeforces Round #666 (Div. 2). The contest starts on 30.08.2020 17:35 (Московское время) and lasts 2 hours. In both divisions, there will be 5 problems for you to solve.

The problems were authored and prepared by Maripium, DatVu, MofK, and me.

We are very grateful and would like to sincerely thank the following people for their assistance in preparing the round:

• 300iq for coordinating our round and for reviewing and helping us with the problems.

• minhtung04042001, TomOwO, EntityIT for their problem ideas and for helping us prepare the problemset.

• codeveloppy, lolicon234, DeadlyCritic, growup974, VietCT, EdBarge, hugopm, Combi, csani, thenymphsofdelphi, Kcidknam, guptapratham42, Umi, quanpham0805, user202729_, Jarjit_sigh, Kuroni, dorijanlendvaj, neko_nyaaaaaaaaaaaaaaaaa, Jatana for testing the problems and providing helpful feedback and suggestions.

• MikeMirzayanov for the amazing platforms Codeforces and Polygon.

Finally, we would like to thank all of you for participating in the contest!

We have spent many months to brainstorm ideas, ended up discarding most of them, and finally chosen our best ideas to compose together this contest, so we hope you will enjoy our round! (and hope the Devil's Number won't haunt our round XD)

Good luck, have fun!

The score distribution will be announced later.

UPD1: Here is the score distribution:

Div. 1: 500 — 1000 — 1750 — 2250 — 2500

Div. 2: 500 — 1000 — 1250 — 1750 — 2500

Hope the long queue disappears soon :'(.

UPD2: Congratulations to the winners:

Div.1

1. tourist
2. Radewoosh
3. aid
4. ksun48
5. Um_nik

Div.2

1. chokottodake
2. 1092515504
3. mickey639
4. matt64
5. zjjcth330

We apologize for the late editorial. Anyway, here it is: Editorial

Hi Codeforces!

The 27th Central-European Olympiad in Informatics takes place from 23-29 August in Nagykanizsa, Hungary. The mirror contests will be held on Aug/26/2020 15:05 (Moscow time) and Aug/28/2020 15:05 (Moscow time) (one day after each official contest day) here on Codeforces in the usual IOI format (it is unrated, the best scoring submission counts, scoreboard is not visible during the contest and there's full feedback throughout the whole contest).

In this year's CEOI, the 4 best problem solvers going to secondary schools of Austria, Croatia, Czech Republic, Germany, Hungary, Italy, Lithuania, Poland, Romania, Slovakia, Slovenia, Switzerland, and The Netherlands will compete. Due to the current situation, unfortunately not everyone will be able to make it onsite, but we hope the offsite participants will also enjoy the event, although without the magical week in Nagykanizsa.

Each day will feature a 5-hour contest. In both contests, you will be given 3 problems with a maximum of 100 points to score on each. Every task has subtasks of varying difficulties, so we hope everyone from newbie to legendary grandmaster will have fun solving the problems (well, legendary grandmasters might want to refer to the instructions here).

We would like to thank MikeMirzayanov for creating Codeforces and Polygon platforms and his invaluable help in making this mirror contest possible, and all of the organizers and sponsors.

We wish you will enjoy the contest just as much as we enjoyed preparing it!

CEOI 2020 Scientific Committee

Gyula Horváth, Attila Gáspár, birka0, busamate, csandras, Emarci15, leanil, mraron, niklaci, uno20001

UPD1: The tutorial for day 1 is here.

UPD2: The tutorial for day 2 is here.

Sorry for the issues with the queue and with the bug interactor of potion causing python solutions to fail. Hope you still enjoyed the problems!

Congratulations to everyone who participated, here are the top 5 combined scores:

• WZYYN 600pts!
• jiangly 600pts!
• mnbvmar 573pts
• wucstdio 573pts
• 142857 530pts

The rest of the combined scores can be found here.

You can also check how would you fare against the official contestants on this ranklist.