Hi, Codeforces!

AIM Tech Codeforces Round 4 will take place on August 24, at 19:35 MSK.

The round is prepared by AIM Tech employees: malcolm, Kostroma, Edvard, yarrr, zemen, gchebanov, VadymKa, zloyplace35, ValenKof, riadwaw 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 qwerty787788, Zlobober, ifsmirnov and AlexFetisov 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 both divisions will be given 5 problems and 2.5 hours to solve them.

Problems C-D-E in the first division have almost the same difficulty, so we advise read all of them.

Scoring in the second division 500-1000-1500-2000-3000, in the first division 500-1000-1750-2250-2250.

We wish you good luck and high frequency rating!

Congratulations to winners!

Div. 1:

yosupo

SpyCheese

DEGwer

W4yneb0t

Um_nik

Div. 2:

epicure

bazsi700

Shavkat_Aminov

Tian.Xie

madn

Strikeskids

Hello Codeforces!

On August 21, 18:05 MSK Educational Codeforces Round 27 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.

The round will be unrated for all users and will be held on extented ACM ICPC rules. After the end of the contest you will have one day 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 prepared by Ivan BledDest Androsov, Vladimir vovuh Petrov, Mike MikeMirzayanov Mirzayanov and me.

Good luck to all participants!

Harbour.Space also has a word for you:

We are delighted to welcome the 2017 ACM-ICPC World Champions, ITMO, to our 2nd Hello Barcelona Programming Bootcamp in collaboration with Moscow Workshops ACM ICPC starting September 27.

All the top Russian teams are coming, including St. Petersburg State University, MIPT, Ural Federal University, Tomsk, Novosibirsk, Saratov and Perm, as well as the world’s top universities such as Waterloo, Central Florida, Hangzhou Dianzi, Singapore, KTH and dozens of others — so far teams from 30 countries have signed up.

The event’s gold sponsor is Sberbank, the biggest commercial and investment bank of Eastern Europe and Russia. Thanks to their support we expect that the top participants will be awarded valuable prizes, alongside high-profile internship and job opportunities.

We can’t wait to see all of you coming to learn, practice and compete on the international stage, smoothing your road towards April World Finals in Beijing.

Ps. Registrations close on September 1.

UPD: Editorial is published

Congratulations to the winners:

Congratulations to the best hackers:

1326 successful hacks and 300 unsuccessful hacks were made in total!

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

Hello everyone!

We're glad to tell you that Moscow International Workshop ACM ICPC will be held at Moscow Institute of Physics and Technology from 7 to 16 of November, 2017. It's a great opportunity for students who want to prepare themselves for the upcoming ACM ICPC season and meet talented programmers from all over the world. The previous camp gathered 28 foreign teams and we surely plan to break this record in the autumn!

The Workshop is held in Dolgoprudny with the support of ITMO University, Lomonosov Moscow State University and Saint-Petersburg State University.

The programme is prepared by Michael Tikhomirov Endagorion, Glev Evstropov GlebsHP and Oleg Snark Christenko, the chief editor of Snarknews.

If you can't attend the whole camp, you can choose a shorten participation from 9 to 16 November. The further information and the cost of participation can be found here. Keep in mind that you need to sign up and fill in the questionnaire before the 1st of November, 2017!

What's more, you сan get to the Workshop for the lower price ($550) if you pay before the 16th of September, 2017. This package includes the academic curriculum, dining, accommodation at the MIPT campus, as well as leisure and sports programs on your days off.

For the further information and the cost of participation you can go here.

Also we have a special offer for dedicated teams: you can participate in the Workshop for free.

While you're thinking about going there or not, you can check the photos or the video from the previous Workshop:

• Our Facebook:https://www.facebook.com/MoscowACM/
• Our Instagram: https://www.instagram.com/acmmipt/

Hi everyone!

We invite you to participate in Codeforces Round #429, wich will take place on August 18th, 18:05MSK.

The problems were set by Fedor Mediocrity Korobeinikov and Vlad totsamyzed Mosko. Special thanks to Alex netman Vistyazh for helping to prepare the round, Alex AlexFetisov Fetisov and Vladislav winger Isenbaev for testing problems, Mike MikeMirzayanov Mirzayanov for Codeforces and Polygon systems.

Participants of both divisions will be given 5 problems and 2 hours to solve them. Scoring will be announced before the round.

We hope you'll enjoy our round! We wish everyone good luck and high rating!

UPD: The scoring is: 500 — 1000 — 1500 — 2000 — 2500. Note, that number of tasks changed from 6 to 5. Also great thanks to Alex Um_nik Danilyuk for testing problems.

UPD: We are really sorry for our awful english statements.

UPD: The round is finished. Sorry for inconvenience again. Congratulations to the winners:

Div1:

1. anta

2. LHiC

3. Radewoosh

4. dreamoon_love_AA

5. ikatanic

Div2:

1. emengdeath2020

2. Svlad_Cjelli

3. denis2111

4. Ehsan22

5. zjt_ioi_2019_ak

UPD: Editorial.

I always was dreaming that one day competitive programming will become a real sport, not just activity for a small group of participants. Why? Because I don't really like working and my tries to do some science were totally unsuccessful. It would be cool to live just by doing what you like. Very childish, I know.

But this blog is not about if competitive programming is important or are there any ways to make it interesting to watch to wider audience. I am just trying to understand, is it currently moving toward real sport or away from it? And I get some mixed signals about that:

Bad signals:

• Big onsites (like GCJ or TCO) seem to cut the number of participants and the amount of prizes.

• Some onsite-finals are turning to online-finals. For example, several years ago we had Russian Code Cup onsite in Russia, currently RCC Finals is online.

• Some big companies are turning away from sport programming (IBM is stopping sponsorship of ACM ICPC).

Good signals:

• Some new finals were created during last years. For example, VK Cup in Russia, SnackDown in India or Code Festival in Japan (for students).

• Some small firms are holding rounds and even created their own little onsite finals.

• Sports programming (not in the financial part) seems to thrive. For instance, there are new platforms for training that were created just in the last year: like atcoder (for international participants) or csacademy.

So, what future will you predict for a competitive programming? Will it become a real sport? Or will it forever be just for fun and for students to get to the interview in a big company?

This article will be presenting a rather classical problem that can be solved using deques, along with an extension that allows you to solve the problem in its more general multi-dimensional case. I have decided to write this article after this discussion on 2D range-minimum query.

The article will be mainly based on this following problem:

You are given an array of numbers A[] of size n and a number k ≤ n. Find the minimum value for each continuous subarray of size k.

We will be now focusing on the linear-time solution to this problem.

Solution:

Consider sweeping from left to right through the array. At every moment we keep a list of "candidates" for minimum values throughout the process. That means that at each moment, you have to add one element to the list and (potentially) remove one element from the list.

The key observation is that, during the sweep line process, we find two values A[i] and A[j] which have i < j and A[i] ≥ A[j], then we can safely discard A[i]. That is because, intuitively, A[j] will continue to "live" in our sweep line more than A[i], and we will never prefer A[i] instead of A[j].

We should now consider pruning all the "useless" values ("useless" as in the statement above). It is easy to see now that doing this will lead to a strictly increasing list of candidates (why?). In this case, the minimum will always be the first element (O(1) query).

In order to insert an element to the back of the pruned candidate list, we will do a stack-like approach of removing all elements that are greater than it, and to erase on element, we just pop the front of the list (if it is not already removed).

This is a well-known approach for finding minima over fixed-size continuous subarrays. I will now present an extensions that allows you to do the same trick in matrices and even multi-dimensional arrays.

The multi-dimensional extension

Problem (2D):

You are given an matrix of numbers A[][] of size n × m and two numbers k ≤ n, l ≤ m. Find the minimum value for each continuous submatrix of size k × l.

Solution:

Consider the matrix as a list of rows. For each row vector of A, use the 1D algorithm to compute the minimum value over all l-length subarrays, and store them in ColMin[][] (obviously, ColMin[][] is now a n × (m - l + 1)-sized matrix).

Now, consider the new matrix as a list of columns. For each column vector of ColMin, use the algorithm to compute the minimum value over all k-length subarrays, and store them in Ans[][] (of size (n - k + 1) × (m - l + 1)).

The Ans[][] is the solution to our problem.

The following picture shows the intutition behind how it works for computing Ans[1][1] for n = 5, m = 7, k = 3, l = 4

The pseudocode is as follows:

def solve_2d(M, k, l):
  column_minima = {} # empty list
  for each row in M.rows:
    # We suppose we have the algorithm that solves
    # the 1D problem
    min_row = solve_1d(row, l)
    column_minima.append_row(min_row)
  
  ans = {}
  for each col in column_minima.cols:
    min_col = solve_1d(col, k)
    ans.append_col(min_col)
  
  return ans

Note that the pseudocode is (deliberately) hiding some extra complexity of extracting rows / columns and adapting the 1D algorithm to the 2D problem, in order to make the understanding of the solution clearer.

The total complexity of the algorithm can be easily deduced to be O(n * m)

Multi-dimensional case analysis

The solution can be extended to an arbitrary order of dimensions. For a d-dimensional matrix of size s₁, s₂, ..., s_d, the time-complexity of the problem is O(d * s₁ * ... * s_d), and the memory complexity is O(s₁ * ... * s_d). This is much better than other algorithms that do the same thing on non-fixed size submatrices (e.g. multi-dimensional RMQ has O(s₁ * ... * s_d * log(s₁) * ... * log(s_d)) time and memory complexity).

Finding the best k minima

The deque approach itself is limited in the sense that it allows you to find only the minimum value over the ranges. But what happens if you want to calculate more that one minimum? We will discuss an approach that I used during a national ACM-style contest where we were able to calculate the best 2 minima, and then argue that you can extend to an arbitrary number of minimum values.

In order to store the lowest 2 values, we will do the following:

Keep 2 deques, namely D1 and D2. Do a similar algorithm of "stack-like popping" on D1 when you add a new element, but instead of discarding elements from D1 when popping, transfer them down to D2 and "stack-like pop" it.

It is easy to see why the lowest 2 elements will always be in one of the two deques. Moreover, there are only 2 cases for the lowest two elements: they are either the first two elements of D1, or the first elements of D1 and D2 subsequently. Checking the case should be an easy thing to do.

The extension to an arbitrary number of minima is, however, not so great, in the sense that the complexity of this approach becomes O(n * k²) for a n-sized array, currently bottlenecked by the number of elements you have to consider in order to find the first k minima. [Maybe you can come up with a cleverer way of doing that?]

Useful links

This is the problem I referred to above: http://www.infoarena.ro/problema/smax. I recommend trying to think it through and implementing it, and translating the statement via Google Translate or equivalent.

Hi everybody!

On Saturday, August 12, 2017, at 14:35 UTC Codeforces Round #428 will be held. As usual, Div.1 participants can join out of competition.

The problems are prepared by me(Sadegh Mahdavi) and NikaraBika(Majid GarooC). Great thanks to Arpa(AmirReza PoorAkhavan) and Livace(Alexey Ilyukhov) for testing the round, KAN(Nikolay Kalinin) for helping us preparing the round and MikeMirzayanov(Mike Mirzayanov) for the Codeforces and Polygon systems.

There will be 5 problems and 2 hours to solve. The scoring will be published later.

The main characters of this round are chosen from the game of thrones series :D

UPD : The scoring is : 500 — 1000 — 1500 — 2000 — 2500

UPD: The judges solutions for problem B incorrectly handled some case, so we are going to rejudge some of the hacks. The pretests are not affected, so the contest is going to be rated.

UPD : The round is finished. Congratulations to winners:

Div 2:

1.mama_budra

2.fatego

3.regmsif

4.Lyra

5.Illyasviel

Div 1:

1.dotorya

2.kmjp

3.I_love_Tanya_Romanova

4.Benq

5.Claris

UPD Editorial

On Sunday, August 6th, 22:00 IST, we will hold the 2nd elimination for IndiaHacks (some more details here). The top 900 individuals who qualified through previous rounds will have the opportunity to participate in this round. The top 25 global participants and top 25 Indian participants will advance to the final round. The link to the contest is here.

After the official round is over, the next morning, on Monday, August 7th, 11:35 IST, we'll hold an unofficial unrated mirror here on Codeforces. This mirror will have ICPC rules. For participants of the official round, please hold off on discussing the problems publicly until after this mirror is over.

I was the author of the problems in this set, and I hope you will enjoy the problems. I would like to thank zemen for testing the set, Arpa for writing editorials, r3gz3n for his help on the HackerEarth side, KAN for helping us set up the mirror contest, and of course MikeMirzayanov for the great Polygon/Codeforces platform.

The round will consist of 6 problems and you will have 3 hours to complete them. Please note that the problems will be randomly arranged in both rounds, since I couldn't figure out how to sort them by difficulty. Be sure to read all the problems.

UPD1: Updated time of official round and posted link to contest.

UPD2: We should have updated the leaderboard to accept solutions that followed the first version of the first problem. We have also increased the number of finalists to 60 total (30 global + 30 indian) based on this new leaderboard.

UPD3: Here is the list of qualifiers. Congratulations to everyone.

Hello Codeforces!

On August 3, 18:05 MSK Educational Codeforces Round 26 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.

The round will be unrated for all users and will be held on extented ACM ICPC rules. After the end of the contest you will have one day 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 prepared by Ivan BledDest Androsov, Alexey Perforator Ripinen, Mike MikeMirzayanov Mirzayanov and me.

Good luck to all participants!

Don’t miss your chance to be a part of this leader board in the next ACM-ICPC World Finals by reserving your spot in the 2nd Hello Barcelona Programming Bootcamp in collaboration with Moscow Workshops ACM ICPC.

Check out the winning statics of Universities that participated in this special training — World Finals 2017 Results.

8 out of 12 prize-winners of the World Finals 2017 participated in Moscow Workshops ACM-ICPC!

Take a look back on our previous "Hello Barcelona ACM-ICPC Bootcamp, in collaboration with Moscow Workshops ACM-ICPC". Students and coaches from all over the globe gathered at our campus to learn from and work with the world’s top programmers, soak in the Barcelona sun, and share in the comradery built within the programming community. Harbour.Space University is looking forward to hosting again, this time at the beautiful and technologically mind bending Media-TIC building.

UPD: Editorial is available

Congratulations to the winners:

Congratulations to the best hackers:

1361 successful hacks and 513 unsuccessful hacks were made in total!

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