Hello Codeforces!

evima, yosupo and I would like you to participate in Codeforces Round #286. It will be held on Sunday, January 18th at 16:00 MSK. Please note that this round starts on unusual time.

Great thanks to Zlobober who helped us prepare this round, Delinur who translated statements into Russian and MikeMirzayanov who created Codeforces and polygon.

This is the 3rd time(following #162 and #263) for me, and the 1st time for evima and yosupo to prepare a Codeforces Round.

Scores of the problems will be

500-1000-1750-1750-2500 for Div.1, and

500-1000-1500-2000-2750 for Div.2.

In this round, you'll help a man named Mr. kitayuta. I hope he will participate :)

The system tests are now over! The top-5 are as follows:

Div.1:

1.ilyakor

2.kcm1700

3.LayCurse

4.RomaWhite

5.TankEngineer

Div.2:

1.Konijntje

2.cpcpc

3.zgzjsxshycxksxhsh

4.Ronnie007

5.sha384

Also, special congrats on Petr, who solved problem E in Div.1, which anyone else could not solve.

Here are the editorials

Round 1 FHC starts in January, 17, at the 18.00 UTC.

Look this and other information here. The top 500 finishers will advance to Round 2, as well as contestant who gets the same number of points as the 500th contestant.

Round 1 will last 24 hours. This is not virtual contest, when you can choose moment when you start during these 24 hours, but 24-hour contest of full value. It is unusually decision. It is clearly that organizers wants all participants to find the convenient time to solve the problems. I hope the problemset we be completed such a way so how many times does participient can spend to solve the problems, 2 hours or 24, will not have a big impact to his result. Perhaps it is assumed that a certain set of problems will be solved by large number of participants, much more than 500, but very few people will be able to solve something else.

(I will glad to see the remarks about translation mistakes)

From 24 February to 6 March 2015, the Moscow Insitute for Physics and Technology will host the sixth Winter Computer Camp 2015 (WCC-2015) in the city of Dolgoprudny. Students of upper grades are welcome to participate.

The program of the camp includes::

• Daily studying contests: preparation to the finals of the Open Olympiad, finals of national programming Olympiads.

• Olympiads in programming and mathematics.

• Lectures by workers of base organizations of FIHT about actual tasks in the IT Industry.

• Lectures by professional scientists on combinatorial mathematics.

• Sports and intellectual games.

How to get to the school:

1. Fill in the registration form.
2. Participate in the online selection rounds for the camp. Pay attention that the selection is conducted thoroughly in compliance with the results of the selection rounds.

Online selection rounds*:

• 18.01.2015 10:00 мск — first round,

• 31.01.2015 15:00 мск — second round,

• 07.02.2015 15:00 мск — third round,

• 08.02.2015 10:00 мск — fourth round

**the dates and time of the rounds can change, follow the information on [our website].(http://it-edu.mipt.ru/zksh2015).

Cost of participation:

• Free participation is guaranteed to the winners and prizers of the final stage of the All-Russian Olympiad for School Students (or CIS Olympiads) in Informatics and Mathematics.

• 5900 for winners and prizers of the ALl-Russian Team Olympiad for School Students in Programming and the Open Olympiad for School Students in Programming.

• 9900 rubles for participants of the final stage of the All-Russian Olympiad for School Students (or CIS Olympiads) in Informatics and Programming

• 18900 rubles for other participants.

For any occuring questions, leave messages on the organizing committee email: [email protected]

The programming committee of Winter Computer Camp 2015 includes: map, Kostroma, Endagorion, DPR-pavlin, ifsmirnov, riadwaw, Babanin_Ivan, Golovanov399, Skird, gchebanov, savinov, Arterm, zemen, kuzmichev_dima, pershik and others.

The organizing committee and the programming committee start its work! See you in February!

Today I want to introduce you some very very useful data structures.

In this lecture, we are trying to improve your data structures skills, stay with us and click on read more.

Hi, Codeforces!

Codeforces Round #285 will be held at 12 January, 12.00 MSK. Problems are authored by me, Evgeny Savinov. This is my first round at Codeforces. I hope that this isn't the last one.

I want to thank sokian and Golovanov399 for help in preparing and testing round, Zlobober for invaluable help in preparing round, AlexFetisov for testing round, Delinur for translating problem statements in English, and of course, MikeMirzayanov for great systems Codeforces and Polygon.

By the way, today(11 January) is MikeMirzayanov's birthday. Happy birthday, Mike!

The round will be for both divisions. Information about score distribution will be posted just before the round starts.

UPD1: Scoring system will be dynamic. Problems will be arranged in ascending expected difficulty order.

UPD2: The editorial can be found here. I'm sorry for the delay.

Mapping a permutation to a number has attracted much social concern. In shortest-path problems, in case a node (or state) is a permutation, we should convert states to integers, in order to BFS or Dijkstra on new graph comfortably. Many coders have known using std::map or trie to do this work. However, both have certain disadvantages. My writing will introduce a new way to solve this problem.

It is hard to write both long and detailed blog. Therefore, you can comment anything which you didn't understand well. I will reply (or update this blog if it is necessary).

To understand the role of mapping a permutation to a number, consider problem POSLOZI from COCI. Our goal is to find the length of the shortest path from a permutation S to an other permutation T using allowed operations. A valid operation is swapping two elements in the permutation. We are given a list of pair (p, q) denote we can swap the element indexed p and the element indexed q. Any other swapping operations are not allowed. A possible strategy is to BFS simultaneously from both S and T.

.

I see lots of programmers write code like this one:

pair<int, int> p;
vector<int> v;
// ...
p = make_pair(3, 4);
v.push_back(4); v.push_back(5);

while you can just do this:

pair<int, int> p;
vector<int> v;
// ...
p = {3, 4};
v = {4, 5};

Hello 2015! Hello Codeforces!

It seems it is time to take stock. Frankly, I was almost feared to start summing statistics of 2014. In 2013 Codeforces showed rapid growth so that it would not be surprising to look bad on the background of 2013. Certainly not! I was pleasantly surprised by the statistics and reports!

Just below is a list of major events and achievements of Codeforces over the year. For you, it's just a list, but please note — every item includes hard work of multi-day Codeforces team, writers of problems, the organizers of contests and tournaments, problem testers and volunteers. Yay! Together we have done all of this:

• introduced Codeforces API
• added (and sometimes improved) all Andrew andrewzta Stankevich contests
• Codeforces supported mode to work as iframe-widget, and Codeforces helped Google to run https://www.calltocode.ie/ for Ireland high school children
• nearly 70 rounds have been hosted for our beloved users
• nearly 450 new and interesting problems were prepared for the contests on the Codeforces platform
• supported new programming languages
• together with CROC was held Coder-Strike 2014
• hosted April Fools Day Contest 2014 from magnificent Maria Nickolas Mykhailova

• together with the jury of Russian Code Cup 2014 held RCC Warmup, who opened the RCC for a wide range of Codeforces users
• Codeforces played a role of press-partner of RCC, ACM-ICPC Finals in Ekaterinburg, Yandex.Algorithm
• we held ZeptoLab Code Rush 2014 (Om Nomes!)
• helped wonderful company RocketFuel to run Rockethon 2014 (on Codeforces)
• hosted MemSQL Start [c] UP 2.0 with the finals the MemSQL office!
• had 11 episodes of Codeforces Trainings Season 02
• together with the jury Bayan Contest held Bayan Contest Warm Up
• updated Polygon (http://polygon.codeforces.com/) introducing an infinite number of small improvements
• supported tutorial in Polygon
• file system queries caching in Polygon has led to significant acceleration of most pages
• together with GridDynamics held GridGames (for Saratov university students)
• broke all records: 6274 registrations on Good Bye 2014!

In this cold January day, I send warm greetings to the Codeforces team and our friends: developers, problem coordinators, problem writers, testers, tireless bloggers and all of you — the knowledge-hungry competitors!

And here is a comparison with previous years of Codeforces. Clearly. In Pictures.

As you know, the C++ language assumes that the programmer is always correct. Therefore C++ compilers don't add additional checks to the program, such as checks for null pointer dereference or out-of-bounds array access. This is good, because C++ programs run as fast as possible, and this is bad, because sometimes we may spend a long time debugging some silly mistake. We would want that the compiler can find such mistakes automatically. And many compilers can! In this post I will show various GCC options that do this. Previously zakharvoit already wrote about this here.

All options that will follow should be added to the GCC command line. In various IDEs you can do it in IDE or compiler settings. Many of the options can also be used with Clang (for example, in Xcode). For MSVC++, I think, there is nothing better than Debug mode and /W4.