Hi everybody.

This change will not probably be among the most popular ones but nevertheless, vote counting rules and voting rules in general are going to change. Why? There are several reasons. The main is that we decided to give up the +2/-1 strategy that used to favor authors. This balance worked fine in the Coderofces's initial stage of development. However, it now leads to overrating many uninformative posts. Moreover, such rating is not valid. Thus, a positive vote and a negative vote now have the same value and the total rating sign now represents the actual attitude of the community members to the post or a comment.

Also, as Codeforces is a professional community of those who is interested in sports programming, it was decided to consider the participants' professional achievements. Thus, the value of a participant's vote now is delapnds on the participant's authority and the authority is determined by the rating. At this point I have to mention that the participant's authority is not influenced by his/her contribution. The votes' values do not differ dramatically: despite the fact that the red participants are 15+ times less that the blue ones, their votes' values differ less than their numbers.

We've introduced additional fraud detection heuristics (some of them had been used on Codeforces). These new strategies make it impossible to cheat in various ways or carry out a grudge, they make reverse engineering systems more complicated and influence some other stuff now and then. The exact details of the vote counting rules are not disclosed but the main principles are given above.

Of course, the change in vote counting will trigger considerable changes in some participants' contributions. I guess that's the indicator of the character of the votes' distribution and the voters' authority. All votes will be recounted that will lead to change in contribution.

MikeMirzayanov

Hi there!

I'm the author of today's CF round.

During the round you'll again assist far away kingdom citizens in solving their everyday problems.

I want to thank Artem Rakhov for invaluable help during the round preparation, Maria Belova for translation of the problems, Mikhail Mirzayanov for excellent CF system and all participants for not leaving this event without your attention.

More AC verdicts and high rating to all of you! gl & hf

Dear friends!

The next competition - Codeforces Round 101 for participants Div2 will take place through a pair of hours, but the others can traditionally participate out of competition. It has been prepared by a small command of authors: me (NALP), Polina Bondarenko (Polichka) and Gerald Agapov (Gerald). There were Artem Rahov (RAD), Maria Belova (Delinur) and Michael Mirzayanov (MikeMirzayanov) with us as always.

It is a little bit about authors: all of us study at the Saratov State University at faculty of Computer Sciences and Information technology on 4th course. Now in free time from competitions and problems we pass the exams :) I’m a 1/3 part from command Saratov SU#2, and Gerald with Polina - 2/3 from Saratov SU#1. We would like our rounds became kind tradition on Codeforces and you have seen us among authors soon again.

We hope that this problems will be pleasant to all participants and everyone will take the deserved high place in the total table. Today you will help Santa Claus, Elf, Vasya and the other characters. All the similarities to real people are casual :)

The points on problems are distributed today in this way: 500-1000-2000-2000-2500

UPD: You can read the tutorial here.

Hello everybody!

I hope you have finished New Year celebrations, or you are ready to make a small break to take part in the anniversary Codeforces Round #100. Round will have place January 4 at 15:00 (UTC). There will be a common contest for participants of the both divisions with the same set of 6 problems. The top 100 participants of the contest receive prize t-shirts.

Score distribution: 500-1000-1500-2000-2500-3000 

Hello

We've scheduled Codeforces Testing Round #4 on January, 3 15:00 (UTC). It will be non-rated event, just to test system before important Codeforces Round #100. The round duration is 1 hour and it will contain just two problems. I do not promise something interesting and exciting, but as a small warm-up will be nice :)

I'll be glad to see you on the round,
MikeMirzayanov

The testing is successfully completed. Thank you for participation. I hope you like this sprint.

Codeforces project congratulates everybody who is into sport programming with New Year! A New Year is not just an increment of a year. Let this day be a starting point to your new achievements, accomplishments and success. Codeforces wishes you to find out by the end of 2012 that all your resolutions have been a success! We wish you to have interesting problems, nice solutions, correct code and more and more of beautiful green messages  Accepted.

As well as in the previous year, we enable handle change for ten days. The handle change is available from the "Settings → Handle" tab from your profile page. Handle change will be disabled on January 10, so don't lose the moment!

Happy New Year, Happy New Coding!
MikeMirzayanov

Hi everybody!

The calendar shows the end of December — the year 2011 is about to end. It was an eventful and exciting time for us. You can find pictures below summarizing 2011 in comparison to 2010. In a nutshell, we've made it! We have grown in all parameters! This result is the shared victory of the well-united team. My special thanks go to the VK (VKontakte) and specifically — to Pavel Durov. The programmers' community and its development really matters to them. I want to thank the authors of the problems: preparing contests is tough and you are a significant help to the Codeforces project and to the whole community.

So, comparing the ending 2011 year and the 2010 year in pictures goes like this:

Codeforces users' growth throughout the Codeforces history (by months)

Hello, everybody.

New Year Holiday is a time of miracles and gifts! Quite by chance the 100th Codeforces round coincided with this wonderful moment.

So, January 4 at 15:00 (UTC) the anniversary Codeforces Round 100 will have place. Yes, we say goodbye to the word Beta in round titles :)

It will be a combined round, that is, participants Div1, Div2 and newcomers will compete with one set of problems. To make it interesting for each participant we plan to expand the round to 6 problems.

The most important thing: the best hundred participants of the 100th round will receive an exclusive Codeforces t-shirt!

Happy new year!
Codeforces Team

Hello.

Today round is prepared by me. My name is Mikhail Tikhomirov, i am fourth grade student at mech.-math. dep. of MSU, also i work as developer-researcher at Yandex.

I want to thank Artem Rakhov (RAD) for valuable help and thoughtful coordination, Maria Belova (Delinur) for great-as-always translating statements into English, and alsoMikeMirzayanov for letting us all get together today. =)

Round will be for both divisions. Every division will have five problems as usual, some of them will be the same, some will be not.

Score distribution:

Div1: 500-1000-2000-2000-2500.

Div2: 500-1000-1500-2000-3000.

Today round is the last round in 2011. I want to thank Codeforces team, everyone who invented, prepared or helped in preparing problems this or past years, and those, who help developing the project. Codeforces now is not just a platform for programming competitions, it is a place where everyone can learn something from another, get a bit of knowledge from more experienced fellows, become more advanced by solving contests and trainings, or just enjoy cool and beautiful problems.

Let's wish the Codeforces project good luck in development next year and long years of existence.

A. Postcards and photos

We will move from the left of string to the right. When we passed the whole string, or in the hands of us have 5 pieces, or current object is different from what we hold in our hands, we remove all the items in the pantry. The answer to the problem is the number of visits to the pantry.

The complexity is O(n).

B. Permutation

We can count the number of integers from 1 to n, which occur in sequence at least once. Then the answer is n minus that number.

The complexity is O(n).

C. History

Denote a[i], b[i] - ends of the i-th event. Let's sort pairs (a[i], b[i]) by a[i] and iterate over all pairs. Denote rg the maximal b[i] from already processed. If current b[i] < rg than we must increment answer by one. If b[i] > rg than we must assign rg by b[i].

The complexity is O(n logn).

D. Palindromes

Let's preprocess array cnt[i][j] - the minimal number of changes tha we must do to make substring from position i to j palindrom. We can easy calc cnt[i][j] with complexity O(n^3). Now we can calculate dynamic programming z[i][j] - minimal number of changes that we can do to split prefix of length i into j palindromes. In begining we must assign z[i][j] = infinity for all (i, j) and assign z[0][0] = 0. If we want to make updates from state (i, j) we must fix the length of j-th palindrom - len. We can update z[i + len][j + 1] by value z[i][j] + cnt[i][i + len - 1]. Answer to the problem is the min(z[n][i]), where n is the length of string and i from range [1, k].

The complexity is O(n^3).

E. Last Chance

Let's replace all vowels by -1 and all consonants by +2. Obviously substring from position i to j is good if sum in the substring [i, j] is nonnegative. Denote this sum by sum[i][j]. Obviously sum[i][j] = p[j + 1] - p[i], where p[i] is the sum of first i elements. Now for all i we want to find maximal j such that j >= i and sum[i][j] >= 0. For this let's sort the array of (p[i], i) and build segment tree on this array by i. Let's iterate over all p[i] in nondescending order. Obsiously for fixed i we have that j = max(index[i]), where index[i] is the index of i-th partial sum in nondescending order and i from range [x, n], where x is the position of the first partial sum with value p[i] in sorted array. Than we must update position i by value of negative infinity and update answer by j - i.

The complexity is O(n logn).