Hello Codeforces!

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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 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.

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 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:

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, growup974, dorijanlendvaj, Osama_Alkhodairy, Mohamed.Sobhy, pikmike, kdjonty31, csani, BledDest, saurabhshadow, postscript, 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. I_love_chickpea
2. Egor
3. jiangly
4. Geothermal
5. dlalswp25
6. antontrygubO_o
7. ashmelev
8. Maksim1744
9. Heltion
10. SSRS_

Div.2

1. gamegamewillwinioi2020
2. altair-chan
3. CongLingDanPaiShang2k5
4. rainboy
5. kissenok
6. Noche_5
7. ghj1222
8. nitinjan06
9. Rchen3
10. Gotz_X

Algorithms Thread Episode 8: Tree Basics

Episode 8 of Algorithms Thread comes out in <90 minutes! This one is a bit more beginner-friendly and covers the following ideas:

• Graph/Tree Diameters
• Binary Lifting
• Tree Flattening with Euler tours

Also, to make sure you have actually learned that stuff, I made a custom Gym set on CodeForces that will last two weeks that hopefully is really good practice for making sure you have learned this stuff. Here is a link to the gym set; it will be available 45 minutes after the video comes out so that people have time to watch the video before starting the set, if they are interested in penalty points. All of the problems in the gym are original to this set (in their flavortext at least, some are simple enough that I'm sure they have appeared in other contests before).

The new gym integration was heavily inspired by Errichto's Matrix Expo set format. Let me know whether it's helpful. I think it might be, but also it's a pretty big time commitment to make it, so whether I keep doing them depends on how helpful they are to people.

If you have any questions or suggestions, feel free to leave them below. I hope you enjoy the problem statements, and I'll leave you all with this:

Solutions

Update:

• Video solutions to all problems

• Github containing solutions to all problems and data generators

Hi!

On Aug/16/2020 17:35 (Moscow time) we will host Codeforces Global Round 10.

This is the fourth round of a 2020 series of Codeforces Global Rounds. The rounds are open and rated for everybody.

The prizes for this round are as follows:

• The top 30 participants get a t-shirt.
• 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.

The prizes for the 6-round series in 2020 are as follows:

• In each round, the top 100 participants get points according to this table.
• The final result for each participant is equal to the sum of the points they got in the four rounds where they placed the highest.
• The top 30 participants over all series get sweatshirts and place certificates.

Thanks to XTX for supporting the global rounds initiative in 2020!

The problems in this round were prepared by KLPP, zscoder, qlf9, golions, MagentaCobra, and me. We would like to give a huge thanks to the following people:

• antontrygubO_o for rejecting most of our problems coordinating our round.
• gamegame, dorijanlendvaj, thenymphsofdelphi, aryanc403, Ari, alireza_kaviani, KonaeAkira, Mn619, AmShZ, smax, galen_colin, cuber_coder, gabrielwu, nvmdava, 300iq, Monogon, Mohamed.Sobhy, maroonrk, kefaa2, pajenegod, Devil, Osama_Alkhodairy, Mohammad_Yasser, Taran_1407, Um_nik, physics0523, hugopm, mohammedehab2002, gonP, tfg, Joker_11, kshitij_sodani, JAPL, duckmoon99, Blastman, growup974, and Antebe for testing and providing invaluable feedback.
• MikeMirzayanov for the Codeforces and Polygon platforms.
• All of you for participating in our contest!

We had a lot of testers as the problemset of the round changed significantly throughout testing! As a result of the huge amount of feedback, we think that we've managed to make the round really high quality and hope that you'll enjoy it :)

You will be given 3 hours to solve 9 problems. The score distribution will be announced at some point in time before the contest starts. Good luck!

UPD: Score distribution:

500 — 750 — 1000 — 1500 — 2000 — 2500 — 3000 — 3500 — 4000

UPD: Editorial

UPD: System tests have finished. We hope you liked the problems! We apologize for the weak pretests on A and B — that was a major oversight on our part. Hopefully you were still able to enjoy the contest regardless :)

Congratulations to the winners!

1. boboniu
2. maroonrk
3. ecnerwala
4. tourist
5. Petr
6. ksun48
7. tmwilliamlin168
8. apiadu
9. whzzt
10. hos.lyric

Hello Codeforces!

On Aug/14/2020 17:35 (Moscow time) Educational Codeforces Round 93 (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:

Congratulations to the best hackers:

85 successful hacks and 696 unsuccessful hacks were made in total!

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

UPD: Editorial is out