Hello Codeforces!

We have a pleasure to invite you to Good Bye 2021, which will take place on Dec/29/2021 18:35 (Moscow time). You will have 2 hours to solve the problems. The round will be rated for participants of both divisions.

The problems for this round were prepared by 300iq, with the help of excellent coordinators KAN and 74TrAkToR.

We would like to thank all the testers, who made this round possible: gamegame, thenymphsofdelphi, ko_osaga, golions, Ashishgup, izban, prabowo, 74TrAkToR, Devil, manish.17, taran_1407, minhcool, AlFlen, Utkarsh.25dec, NemanjaSo2005, wxhtzdy, ajit, mnaeraxr, Scrubpai, YashDwivedi, eatmore!

And of course, MikeMirzayanov for great platforms Codeforces and Polygon.

This round is supported by NEAR, a company founded by former competitor AlexSkidanov. NEAR is built by many prominent competitive programmers, including twice ICPC champion eatmore and GCJ and TCO winner Egor.

The participants who end up in the first 255 positions will receive prizes. The participant on the first place will receive Ⓝ128, the next two participants will receive Ⓝ64, the next four participants will receive Ⓝ32, etc.

NEAR is a modern blockchain protocol and a development platform. NEAR applications have digital assets as a first class concept, and are hosted in a way that anyone can ascertain the correctness of their execution. This allows building applications that by design provide users with ownership over their assets, data and the power of governance.

If you want to try building on top of NEAR, join Metabuild, a hackathon running until February with $1M in prizes: https://metabuild.devpost.com/

We hope you will enjoy the problem set! Good luck!

Congratulations the winners!

1. tourist
2. ecnerwala
3. ksun48
4. Radewoosh
5. Benq

UPD: Editorial

I've created an NFT.

I'll have a private one-hour long call with the person who ends up owning it (if you want, you can also try to resell the NFT instead of claiming the private lesson, I don't really care).

The price is not fixed, feel free to make any offers on the OpenSea. The instructions for the owner are listed in the unlockable conent.

We invite you to participate in CodeChef’s December Long Challenge, this Friday, 4th December, from 15:00 IST onwards. The contest will be open for 10 days i.e. until 14th December.

Also, if you have some original and engaging problem ideas, and you’re interested in them being used in CodeChef's contests, you can share them here. Joining me on the problem setting panel are:

• Setters: Ildar 300iq Gainullin, Alexander scanhex Morozov, Ivan isaf27 Safonov, Anton antontrygubO_o Trygub, Md. Reyad Fiction Hossain
• Admin : Ildar 300iq Gainullin
• Tester: Alexander scanhex Morozov
• Editorialist: Ajit Sharma ajit Kasturi
• Video Editorialists: Chirayu [user: chirayu_jain98] Jain, Prachi agarwal19 Agarwal, Darshan darshancool25 Lokhande, Yashodhan Agnihotri, Bharat Singla, Aryan Aggu_01000101 Agarwala, Radoslav radoslav11 Dimitrov
• Statement Verifier: Jakub Xellos Safin
• Mandarin Translator: Hanlin Ioser Ren
• Vietnamese Translator: Team VNOI
• Russian Translator: Fedor Mediocrity Korobeinikov
• Bengali Translator: Mohammad solaimanope Solaiman
• Hindi Translator: Akash Shrivastava akashsrivastava143

Prizes:

Top 20 performers in the Indian category and top 10 performers in the Global category will get CodeChef laddus, with which the winners can claim cool CodeChef goodies. First to solve each problem except challenge — 100 laddus. Know more here

The video editorials of the problems will be available on our YouTube channel as soon as the contest ends. Subscribe to get notifications about our new editorials.

Good Luck!
Hope to see you participating!

We invite you to participate in CodeChef’s November Lunchtime, this Saturday, 28th November, from 7:30 pm to 10:30 pm IST onwards 3 hours, 5 problems.

We will also be hosting two live problem discussion sessions on Sunday (29th) from 5pm to 6:30pm IST (first 4 problems) and on Monday (30th) from 5pm to 6:30pm IST (last 3 problems), where our panelist, rajarshi_basu will discuss the Lunchtime problems. Find more details here.

Also, if you have some original and engaging problem ideas, and you’re interested in them being used in CodeChef's contests, you can share them here.

Joining me on the problem setting panel are:

• Setter & Admin: Ildar 300iq Gainullin
• Tester: Nikolay Nebuchadnezzar Budin
• Editorialist: Alei Morphy Reyes
• Video Editorialists: Chirayu Jain, Prachi agarwal19 Agarwal, Darshan darshancool25 Lokhande, Yashodhan ArnoldShivajinagar Agnihotri, Bharat Singlai, Aryan Aggu_01000101 Agarwala, Radoslav radoslav11 Dimitrov
• Statement Verifier: Jakub Xellos Safin
• Mandarin Translator: Qingchuan UoA_ZQC Zhang
• Vietnamese Translator: Team VNOI
• Russian Translator: Fedor Mediocrity Korobeinikov
• Bengali Translator: Mohammad solaimanope Solaiman
• Hindi Translator: Akash Shrivastava

Prizes: Top 10 Indian and top 10 Global school students from ranklist will receive certificates and CodeChef laddus, with which they can claim cool CodeChef goodies. Know more here. The video editorials of the problems will be available on our YouTube channel as soon as the contest ends. Subscribe to get notifications about our new editorials.

Good luck and have fun!

If you are a school student and love programming competitions, enjoy solving mathematical and logical problems, designing, and implementing algorithms and data structures – we invite you to participate in the International Olympiad in Informatics — Innopolis Open.

The contest is one of several international Olympiads annually held by Innopolis University. The best high school students from different countries compete in the prestigious competition to sharpen their informatics skills.

3 steps to win!

• Sign up until November 22, 2020 choosing a category: Informatics. For Russian speaking countries, the registration is here
• Pass the Qualification Online Round. There will be 2 independent online contests, you can participate in both contests. The best result of two contests will be taken to get to the Final Stage.
• • 1st Qualification Online Round: November 22, 2020 at 10:00 a.m. (UTC +3), duration — 5 hours
• • 2nd Qualification Online Round: December 13, 2020 at 3:00 p.m. (UTC +3), duration — 5 hours

• Be on the Final Stage at Innopolis University (we hope it would be on-site, not online) on February 20-21, 2021.

We invite you to participate in CodeChef’s October Long Challenge, this Friday, 2nd October, from 15:00 IST onwards. The contest will be open for 10 days i.e. until 12th October.

Also, if you have some original and engaging problem ideas, and you’re interested in them being used in CodeChef's contests, you can share them here. Joining me on the problem setting panel are:

• Setters: Ildar 300iq Gainullin, Aleksandr enoone Abelyan, Ivan isaf27 Safonov, Chaithanya Dragonado Shyam, John Smith, Md. Reyad Fiction Hossain
• Admin: Ildar 300iq Gainullin
• Editorialist: Jelmer Firet, Srikkanth srikkanthr
• Tester: Radoslav radoslav11 Dimitrov
• Video Editorialists: Chirayu Chirayu Jain, Prachi agarwal19 Agarwal, Darshan darshancool25 Lokhande, Mani manijuana Tyagi, Divesh divesh2201 Thakker
• Statement Verifier: Jakub Xellos Safin
• Mandarin Translator: Hanlin Ioser Ren
• Vietnamese Translator: Team VNOI
• Russian Translator: Fedor Mediocrity Korobeinikov
• Bengali Translator: Mohammad solaimanope Solaiman
• Hindi Translator: Akash Shrivastava

Prizes:

P.S: For October Long Challenge, we won't be having any tie-break Challenge problem. So, instead of giving the usual cash prizes to the top-ranked users, we will give laddus, which help in the fair distribution of reward. For the Indian rank list, we'll distribute 5000 laddus amongst all the top-scorers. For the Global rank list, we'll distribute 13000 laddus amongst all the top-scorers. The top 20 performers in the Indian category and top 10 performers in the Global category will get CodeChef laddus, with which the winners can claim cool CodeChef goodies. First to solve each problem except challenge — 100 laddus. Know more here

Good Luck!
Hope to see you participating!

Hello Codeforces!

I became a coordinator of CodeChef Lunchtimes, and we are in need of high-quality proposals.

We hope to get rid of the 'CodeChef has low-quality problems' label.

So we would be grateful if you can provide a problem set that will work fine for the IOI-style contest! (Note that your problems should not necessarily follow the IOI syllabus).

You can read about compensation and the required difficulty of problems here.

Proposals for Lunchtimes should be submitted to the following e-mail: contest.admin2@codechef.com

P.S. If you have hard problems for CodeChef Long Challenges, you can send them here

I won't hold any coding streams in May.

Hi!

On Mar/19/2020 17:35 (Moscow time) we will host Codeforces Global Round 7.

It is the first round of a 2020 series of Codeforces Global Rounds. The rounds are open for everybody, the rating will be updated for everybody.

The prizes for this round:

• 30 best 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:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

The problems of this round were developed by isaf27 and me. Thanks to the testers mohammedehab2002, taran_1407, Aleks5d, Endagorion, 74TrAkToR, HIR180, dlwocks31,ToTheMoon, coyorkdow, Tzak, DomiKo, JustasLe, Hyado, Nemo, Merchant, Jatana, and (language corrector!) caoash.

Thanks to XTX, which in 2020 supported the global rounds initiative!

Good luck!

UPD: Score distribution: 500 1000 1000 (1000-1000) 2500 (2000-1500) 4000

UPD: Editorial!

You are given a string $$$s$$$, and for each $$$r$$$ you need to find the largest $$$L_r$$$, such that $$$s[r - L_r + 1 \ldots r]$$$ is a palindrome.

It is possible to solve this problem with the eertree or with Manacher's algorithm with some data structures, but I will describe a simpler way.

You will need some black box, that for any substring $$$s[l \ldots r]$$$ can check in $$$\mathcal{O}{(1)}$$$ if it is a palindrome. The easiest such black box is a polynomial hash, but also you can precalculate stuff from Manacher's algorithm and then check that $$$\frac{(l+r)}{2}$$$ is a middle of a long enough palindrome.

The key fact here is that $$$L_i \leq L_{i-1} + 2$$$, because if $$$s[l \ldots r]$$$ is a palindrome, then $$$s[l+1 \ldots r-1]$$$ is a palindrome too.

With this observation, we can use our black box to find the required values!

Let's assume that you already know $$$L_1, L_2, \ldots, L_{i-1}$$$ and we want to calculate $$$L_i$$$.

Starting from $$$L_i = L_{i-1}+2$$$, decrease $$$L_i$$$ while $$$s[i - L_i + 1 \ldots i]$$$ is not a palindrome.

The number of black box operations of this algorithm is $$$\sum{(L_{i-1} + 2 - L_i)}$$$ $$$\leq 2 n$$$.

Hello everyone, this winter at Petrozavodsk was my third (Petrozavodsk) contest, now it is loaded on the gym.

Contest link.

Virtual participation link.

Thanks to the testers MiracleFaFa, ko_osaga, Radewoosh, ksun48, xiaowuc1, ainta, molamola., Endagorion, antontrygubO_o, slime

UPD: Editorial link

The elimination round of Engine VK Cup 2019-2020 will take place at Feb/23/2020 19:05 (Moscow time). This contest is intended for people who qualified earlier.

As usual, there will be two parallel rounds, one for each division, for everybody who can't take part in official round.

Thanks lperovskaya, MikeMirzayanov, PavelKunyavskiy, izban, Kurpilyansky, YakutovDmitriy, 300iq for preparing the contest.

All three rounds will last 2 hours 30 minutes, and they will be rated. They will have six tasks, and you will be able to see the score distribution when the round starts.

Hello, Codeforces community!

I'm glad to invite you to Codeforces Round #609 (Div. 1) and Codeforces Round #609 (Div. 2), which will be held on Dec/21/2019 14:05 (Moscow time). The round will be rated for both divisions.

The problems were taken (mostly) from the ByteDance — Moscow Workshops Online Contest, that's happening at the same time. They were prepared by myself and tested by MiracleFaFa, Claris, quailty, jiry_2 (camp TA team), and gamegame, isaf27, tmwilliamlin168, mango_lassi, WNG, Lewin, sas4eka, UselessDev, Aleks5d,MrDindows.

ByteDance is a technology company operating a range of content platforms that inform, educate, entertain and inspire people across languages, cultures, and geographies. ByteDance has partnered with Moscow Workshops ICPC and Codeforces to organize a top tier and exclusive training camp for the International Collegiate Programming Contest. The upcoming Programming Camp will be held in Beijing from February 10th to 16th, 2020.

ByteDance — Moscow Workshops Online Contest is an opportunity to participate as teams in this camp.

You can find more information about this training camp, including registration and prizes at https://programcamp.bytedance.com/.

UPD: Editorial

Hello, everyone!

I think that you often have something to ask or discuss advanced CP topics or some OpenCup rounds or hard problems, and it is not worth creating new codeforces blog, but current popular places with CP chats (i.e OJ chats, or discord, or some country-local chats) are too local and don't have too many people with who you can divide your discussion.

So, I come up with the idea and talked with some of my friends, that it would be cool to create telegram chat with some lower bound on the rating to join.

Now I suggest a lower bound 2600 on rating.

I. e. if you want to join the chat, and if your MAX rating is at least 2600 (so if at some point you had IGM rank), you can write me your telegram login in personal messages, and I will add you to the chat.

Of course, the lower bound may be adjusted later. And if you have suggestions about the proper places for creating this chat, you can write them in comments :) (don't worry, telegram is very convenient!)

Hello everyone, this summer at Petrozavodsk was my second ICPC contest, now it will be held as OpenCup round.

XX Open Cup Grand Prix of Kazan takes place on Sunday, September 8, 2019, at 11:00 AM Moscow time

The link to the contest. You need an Open Cup login to participate.

To ensure Codeforces traditions, I will say thanks to the testers Benq, MiracleFaFa, whzzt, sunset, TLE, ko_osaga, rushcheyo, jiry_2, gamegame, slime. Also thanks to izban with help in tasks choosing.

UPD: Now the contest is loaded to the gym!

Hello everyone, this winter at Petrozavodsk was my first ICPC contest, now it is loaded on the gym.

Contest link.

Virtual participation link.

To ensure Codeforces traditions, I will say thanks to the testers TLE, whzzt, sunset, MiracleFaFa, isaf27. Also thanks to izban with help in tasks choosing.

And of course, thanks to MikeMirzayanov for Codeforces and Polygon platforms and help with loading this contest to the gym.

Hi!

I'm glad to invite you to take part in Codeforces Round #562 (Div. 1) and Codeforces Round #562 (Div. 2), they will be held in May/26/2019 18:35 (Moscow time). The round will be rated for both divisions (^人^).

Participants in each division will be offered five problems and two hours to solve them.

The problems were written and prepared by me. Thanks to KAN for his help with the round, to sunset, TLE, Sooke, isaf27, Lewin, Aleks5d and wrg0ababd for testing and task discussing! Also, thanks to MikeMirzayanov for amazing systems Codeforces and Polygon!

Congratulations the winners!

Div1:

1) kiwikiwi

2) OnionPringles

3) Errichto

4) maroonrk

5) Um_nik

Div2:

1) Szoboszlai10

2) lelolas

3) ndmitrovic

4) prick

5) Stardust

Editorial

Hi!

At Apr/06/2019 14:35 (Moscow time) we will host Codeforces Global Round 2.

It is the second round of a new series of Codeforces Global Rounds supported by XTX Markets. The rounds are open for everybody, the rating will be updated for everybody.

The prizes for this round:

• 30 best 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 2019:

• In each round top-100 participants get points according to the table.
• The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
• The best 20 participants over all series get sweatshirts and place certificates.

The problems of this round were developed by a team of authors: 300iq, cyand1317, Aleks5d, RDDCCD, KAN, gen.

Thanks KAN for his help in the round's coordination, and also isaf27, Lewin, ----------, Errichto, arsijo, cdkrot for testing the round!

Удачи!

Congratulations the winners!

1) ecnerwala

2) tourist

3) Um_nik

4) Endagorion

5) Petr

Editorial.

Hello, Codeforces community!

Today will be a new useless stream from me, I will write UOJ 90. (I don't know how long it will take to write this problem, I've just found Los_Angelos_Laycurse comment where he said that he has 1000+ lines).

I hope I will start around 22:35 UTC+3 (so it's 50 minutes from the moment of writing the post).

Now my stream will have a webcam and donate notifications!

https://www.twitch.tv/ccciq