Hello, codeforces!

It's time to continue the series of Polish tasks. I've decided to write about my own task one more time. Its name is "cook" (you can submit here). The task isn't very hard, but it uses cute (in my opinion) trick. The statement goes as follows:

There is a cook in a restaurant. He has n (1 ≤ n ≤ 10⁶) orders which he must fill. Every order is a piece of paper, and all orders are speared on a spindle (sharp stick with pierced pieces of paper) in a fixed order which cannot be changed. Normal cook would just take orders one by one from the top of the spindle and fill them in this order, but the cook in this task has supernatural cooking powers and can combine orders to fill them faster. In particular, if at some moment there are k out of n orders still on the spindle, he can choose one of three options:

 —  He can take the topmost piece of paper and fill this order in time one(k).

 —  If k > 1, he can take two topmost pieces of paper and fill both orders in total time two(k).

 —  If k > 1, he can take topmost pieces of paper and fill these orders in total time half(k).

This task is interactive, so you should communicate with the library and ask it for values of one, two and half. You can ask as many times as you want and assume that the library works in negligible time, so your only limit is the time limit. Please, note, that when k = 2 functions one and half both fills only one order, but they might take different amounts of time. This same applies to other similar situations.

Also, the cook has an energy level, initially equal to e (0 ≤ e ≤ 10⁶). He likes preparing food without any tricks, so whenever he uses the first option his energy increases by one. However, his half combo tires him very much, thus each time when he chooses the third option his energy decreases by one. Cook's energy cannot drop below zero at any time. Of course, we are asked about the minimum amount of time in which cook can finish all orders. Final energy level doesn't matter.

Last thing: memory limit is unusual because it's equal to 8MB.

Hi, Codeforces!

AIM Tech Codeforces Round 5 will take place on Aug/27/2018 19:35 (Moscow time).

The round is prepared by AIM Tech employees: Kostroma, riadwaw, Edvard, yarrr, zemen, Errichto, malcolm, gchebanov, VadymKa and zeliboba.

Round will take place during Petrozavodsk Summer Camp, which is sponsored by our company.

Thanks to Mike Mirzayanov(MikeMirzayanov) for brilliant platforms Polygon and Codeforces and problem coordinator Nikolay Kalinin (KAN). Many thanks to Golovanov399, Arterm, winger for round testing!

Our company specialises in proprietary trading, the key concepts in our work are big data, low latency and high frequency. Our team mainly consists of graduates from the Moscow State University (MSU) and Moscow Institute of Physics and Technology (MIPT). You could read more on our website aimtech.com.

Participants of combined round will be given 8 problems and 2:15 to solve them.

Last three problems have almost the same difficulty, so we advise read all of them.

Prizes from round 502 in memory of Leopoldo Taravilse will be distributed in this round.

Top-25 will get 100$ each, following 46 will get 50$ each.

Scoring 500-750-1250-2000-2500-3250-3250-3500

We wish you good luck and high frequency rating!

Thank you for participation, congratulations to the winners!

1. LHiC
2. jqdai0815
3. bmerry
4. Um_nik
5. Egor
6. Benq
7. tqyaaaaang
8. DearMargaret
9. Marcin_smu
10. Swistakk

Editorial

Short editorial by bmerry

Information about prizes and analysis will be published later.

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Hello!

Codeforces Round 506 (Div. 3) will start at Aug/24/2018 17:50 (Moscow time). You will be offered 6 or 7 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. Probably, participants from the first division will not be at all interested by this problems. And for 1600-1899 the problems will be too easy. 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 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 Mikhail awoo Piklyaev, Maksim Neon Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round.

Good luck!

I also would like to say that participants who will submit wrong solutions on purpose and hack them afterwards (example) will not be shown in the hacking leaders table.

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UPD1:

Congratulations to the winners:

Congratulations to the best hackers:

1217 successful hacks and 926 unsuccessful hacks were made in total!

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

UPD2: Editorial is published.

NOTE : Knowledge of Binary Indexed Trees is a prerequisite.

Problem Statement

Assume we need to solve the following problem. We have an array, A of length N with only non-negative values. We want to perform the following operations on this array:

1. Update value at a given position

2. Compute prefix sum of A upto i, i ≤ N

3. Search for a prefix sum (something like a lower_bound in the prefix sums array of A)

November 6–13, 2018 the boot camp for competitive programming Moscow International Workshop ICPC will be held at MIPT. We invite students to participate from all over the World!

The International workshop helps students to prepare for the regional contests and World Finals of the ICPC championship.

The official language of educational program is English. The trainings include every-day contests, tasks analysis and lectures from coaches from former participants and winners of ICPC and the best Russian universities professors. Program includes interesting entertainment program in a free time. Training will be conducted in two divisions:

Division A is designed to prepare students to participate and win medals in the next ICPC World Finals.

Division B is designed to help teams prepare for the regional and international competitions.

Contests are moderated by Oleg Khristenko: coordinator of the Pankratiev Open Cup and the main editor of Snarknews. Heads of Programming Community are Mikhail Tikhomirov and Gleb Evstropov.

The sooner the payment is made, the lower the cost of participation become. Citizens of the Eurasian Economic Union (EAEU) countries may pay in Rubles and others in American Dollars. This cost includes the academic curriculum, catering, accommodation on the MIPT campus, as well as excursions, sportive activities, collective games in the days-offs.

Cost per person: $630 USD until September 21. Two teams have an opportunity to take part in the workshop for free – learn more about it on the site.

Register for Moscow International Workshop ICPC and win!

Hello, codeforces!

This time I've decided to choose a task from my own contest which took place last April and was known as the Grand Prix of Poland. If you want to write this contest virtually in the future, then consider not reading this blog. If you've participated in this contest and maybe even solved this task, then anyway I recommend reading it, cause this task has many very different solutions, each of them being very interesting (in my opinion). It's also a reason why this blog is longer than previous ones.

I'll write about task C "cutting tree" (not uploaded to the ejudge yet :/). The statement goes as follows:

You are given a tree with n vertices (1 ≤ n ≤ 2·10⁵). The task is to calculate f(k) for each integer k from the range [1, n] where f(k) is defined as the maximum number of connected components of size k which we can "cut off" from the tree. A connected component of size k is a set of k vertices such that it's possible to traverse in the tree between any pair of these vertices using only vertices from this set. Chosen components are not allowed to intersect, so each vertex must belong to at most one component.

#IjustWantContribution

It seems there isn't any blog about Berlekamp-Massey Algorithm around here, so I decided to go on a try. :P

Acknowledgement: Hats off to matthew99 for introducing this algorithm.

What is 'linear recurrence'?

Assuming there is a (probably infinity) sequence a₀, a₁...a_n - 1, we call this sequence satisfies a linear recurrence relation p₁, p₂...p_m, iff . (Obviously, if m ≥ n any p can do :P)

How to calculate k-th term of a linear recurrence?

For a polynomial , we define .

Obviously G satisfies G(f) ± G(g) = G(f ± g).

Because , if we let , then G(f) = 0. Also G(fx), G(fx²)... = 0. So G(fg) = 0 (g is any polynomial).

What we want is G(x^k). Because G(f⌊ x^k / f⌋) = 0, then .

Hello.

On Aug/19/2018 16:35 (Moscow time), Codeforces Round #505 takes place. This is a paired round to #504.

Some problems are taken from VK Cup 2018 Finals (ashmelev, Errichto, Lewin) and some are proposed by me. I'd like to thank my fellows — Dima (cdkrot) who is actually coordinating this round, Kolya (KAN) who brought me here, and also Grisha (vintage_Vlad_Makeev) and Ildar (300iq) just for being nice.

I'd also like to express my gratitude to MikeMirzayanov for multiple bug fixes and awesome Codeforces!

There will be seven problems will the following scoring:
500 — 1000 — 1500 — 1750 — 2250 — 2750 — 3500

UPD. The system testing is over. Editorial (with problem E now)

Congratz to the winners!

1. Swistakk (after all these years, yay)
2. DearMargaret
3. Kostroma
4. Benq
5. AwD
6. xumingkuan
7. webmaster
8. TLE
9. Egor
10. kriii

As in the previous round, thanks to VK social network, GP30 scores will be distributed among the best participants.

Participants are sorted by sum of points for both rounds (if the participant did not participate in one of the rounds, the points scored for it are assumed to be equal to zero), with the maximum time for both rounds from the beginning of the round to the last submission that passed the pretests as tie-break.

Let me remind you that top 10 participants with respect to GP30 score will receive a plush Persik.

Hope you enjoy. Good luck and have fun!

On Aug/18/2018 17:35 (Moscow time) Educational Codeforces Round 49 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 extented ACM 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 7 problems and 2 hours to solve them.

The problems were invented and prepared by Vladimir vovuh Petrov, Adilbek adedalic Dalabaev, Ivan BledDest Androsov, Mike MikeMirzayanov Mirzyanov and me.

Good luck to all participants!

Our friends at Harbour.Space also have a message for you:

Hi Codeforces!

Our Hello Barcelona Programming Bootcamp will be kicking off next month from Sept 26 — Oct 4, and we’d love to see you there!

The boot camp will once again feature the all-time greats Mike MikeMirzayanov Mirzayanov, Andrey andrewzta Stankevich, Michael Endagorion Tikhomirov, Gleb GlebsHP Evstropov, Artem VArtem Vasilyev, Ivan ifsmirnov Smirnov and other world renowned Russian coaches to train the participants.

You will be competing and learning side by side with some of the greatest teams in the world, while learning from the best coaches the ICPC has to offer.

Be sure to register before September 1st so everyone has time to get visas if needed, and of course for the Early Bird Discount of 5% or the Loyalty Discount* of 20% off registration for the boot camp!

*The loyalty discount is offered to universities and individual participants that took part in Hello Barcelona Bootcamps and Moscow Workshops ICPC.

Learn more about Barcelona ICPC Bootcamp

You can ask any questions by email: [email protected]

Congratulations to the winners:

Congratulations to the best hackers:

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

UPD: The editorial is posted