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, JettyOller, Monogon, antontrygubO_o and hugopm.

We would like to thank:

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

• gamegame for always being nice and supportive.

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

• Monogon for making the author list more colorful.

• redblossom, thenymphsofdelphi, qlf9, mcfr, Tlatoani, aohkgnadnart_, McDic, cup_of_tea, bestial-42-centroids, 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 nooinenoojno Stepanova, Mikhail awoo Piklyaev, Maksim Neon 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 nuipojaluista, 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 Aug/30/2020 17:35 (Moscow time) and lasts 2 hours. In both divisions, there will be 5 problems for you to solve.

The problems were authored and prepared by JettyOller, 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, SeehtEntity for their problem ideas and for helping us prepare the problemset.

• EternalFire, SucculentLover, DeadlyCritic, coderz189, VietCT, EdBarge, hugopm, Combi, TechNite, thenymphsofdelphi, mdinh, Tech.Maniac, 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. The_Noble_Brahman_Bison
3. mickey639
4. matt64
5. Kai29

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 26.08.2020 15:05 (Московское время) and 28.08.2020 15:05 (Московское время) (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, lacito, 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.

Hello Codeforces!

On Aug/25/2020 17:35 (Moscow time) Educational Codeforces Round 94 (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 Neon 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:

Congratulations to the best hackers:

401 successful hacks and 778 unsuccessful hacks were made in total!

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

UPD: Editorial is out

Hello Codeforces!

New lesson in English EDU section. This time I will talk about binary search.

More about EDU section you can read in this post.

Hope it will be helpful, enjoy!

Hello Codeforces!

I'm glad to invite you to participate in Codeforces Round 665 (Div. 2) taking place on Aug/21/2020 17:35 (Moscow time). The round is rated for users whose rating is lower than 2100.

All problems were mainly created and prepared by me, and I'd also like to thank:

• adedalic for coordinating and reviewing the round,

• AmShZ, coderz189, dorijanlendvaj, Osama_Alkhodairy, Mohamed.Sobhy, awoo, kdjonty31, TechNite, BledDest, saurabhyadavz, Retired_cherry, errorgorn for testing the problems and giving feedback,

• MikeMirzayanov for Codeforces and Polygon platforms,

• All users for participating in the contest!

You will be given 6 problems and 2 hours to solve them. The score distribution will be announced closer to the start of the round.

Good luck, and have fun!

UPD1 : Score distribution is 500 — 1000 — 1500 — 1750 — 2500 — 2500.

UPD2 : Editorial

UPD3 : System testing has finished. We hope you enjoyed the contest :)

UPD4 : Congratulations to the winners!

Div.1 (Out of competition)

1. tribute_to_Ukraine_2022
2. Egor
3. jiangly
4. Geothermal
5. dlalswp25
6. antontrygubO_o
7. ashmelev
8. Maksim1744
9. Sugar_fan
10. SSRS_

Div.2

1. gamegamewillwinioi2021
2. altair-chan
3. Isouau
4. rainboy
5. kissenok
6. Noche_5
7. ghj1222
8. nitinjan06
9. Rchen3
10. Gotz_X