Блог пользователя Zlobober

Автор Zlobober, 7 лет назад, По-русски
Таблица результатов
Краткий обзор задач

SPOILER ALERT: в комментариях и теле этого поста, скорее всего, будут обсуждаться задачи соревнования, поэтому если вы участвуете в каком-либо онлайн-зеркале, либо хотите порешать задачи самостоятельно, будьте к этому готовы.


05:00: В итоге Вова на последнем часу сдал вторую задачу на сотню, чем поднял себя до 21 места. Далее следуют Денис на 28, Егор на 36 и Саша на 54 местах. Это очень хороший результат, у всех без исключения есть шансы на золото, для чего мальчикам надо сохранить имеющийся темп или чуть-чуть его улучшить, а Саше попытаться отыграться за счёт какой-нибудь задачи во второй день. До встречи во вторник!

04:24: У всех ребят, кроме voidmax, приличный набор подзадач по последней задаче. Надеюсь, Вова сейчас закроет эту дырку и поднимется мест на 30-40.

03:29 Саша сдаёт посденюю на 39 и поднимается ещё вверх. Сейчас бы ей сдать на причный балл, наконец, novruz, и тогда она окажется вообще в золоте.

03:07 arsijo сдаёт задачу Wiring на полный сдал, в результате чего поднимается на второе место.

02:46 Egor.Lifar практически добил nowruz, у него 92 и это текущий рекорд по этой задаче. Denisson чуть-чуть улучшил свой результат по trains и сейчас находится на пятом месте.

02:40 Полтура позади. У yutaka1999 появились 20 баллов по Nowruz, кажется, добить их до 80+ для него это дело техники. У Саши, тем временем, появились 11 по последней. Ей бы сейчас переключиться на первую и набрать халявные баллы — это может сильно придать уверенность на второй половине тура.

02:22 Egor.Lifar досдал несколько тестов по Nowruz и поднялся на 32 место. При этом у Егора есть ещё несколько тестов, которые он не сабмитил вообще. За ним следом voidmax сдаёт вторую задачу на 7 баллов Вова.

02:09 yutaka1999 полностью решил wiring и (внезапно) train и лидирует почти на сто баллов.

02:04 В топ-3 попадает arsijo, набравший 84 балла по первой и 30 баллов по второй. У наших ничего принципиально не меняется, только Денис сдаёт вторую на 13, и Егор принимается за первую.

01:31 Ничего интересного не происходит. По задаче train рекордный балл у китайского участника consecutivelimit, он решил все группы, кроме четвертой и последней (которая стоит, к слову, 51 балл).

01:16 ВНЕЗАПНО появилось три сотни по wiring. Для полного решения нужно сперва выписать квадратную динамику, после чего применить какую-то из стандартных оптимизаций.

01:10 voidmax почти догоняет Denisson по первой задаче, сейчас они 8 и 2 места соответственно. Интересно, что Вова сразу послал все тесты в среднем на 6.5 баллов, тогда как Денис предпочитал сдавать по одному тесту, но со сравнительно более высоким средним баллом. Для Дениса сейчас самое время оставить свои решения в фоне искать ответ получше и заняться другими задачами.

01:05 Тем временем, у всех российских школьников уже не ноль: voidmax тоже бросился на nowruz, сдав на максимально возможный балл один из тестов, а demon1999 и Egor.Lifar предпочли задачу wiring, набрав по ней по 13 баллов.

01:00 Denisson продолжает продвигаться по nowruz, заслав еще два теста на примерно 9 баллов из 10. Сложно сказать, что у него происходит, но решать все тесты индивидуально может оказаться слишком затратно по времени. Уже четыре участника имеют по этой задаче порядка 80 баллов. В топ10 из дружественных сборных замечены Денис, kefaa и saba_tavdgiridze.

00:58 Egor.Lifar получил 13 баллов по wiring, это группа на одну простую формулу, которая тем не менее очень важна для полного решения.

00:55 Иранский участник ITDOI уже имеет больше 80 баллов по nowruz и с большим отрывом лидирует. По остальным задачам пока открыты только простые группы.

00:53 Если я правильно понимаю, то зарегистрированных участников 312 человек, а значит, можно легко посчитать количество медалей каждого вида, зная, что они соотносятся в пропорции 1:2:3:6. Таким образом, 26 место это последняя золотая медаль, 78 место это последняя серебряная и 156 место это последняя бронзовая.

00:46 Denisson открыл счёт среди участников нашей сборной, начав сдавать задачу nowruz. Эта задача примечательна не только тем, что в ней partial scoring, но также тем, что это первая с 2006 2010 года задача с открытыми тестами на IOI. Как видно из таблицы результатов, по ней можно довольно быстро набрать неплохой балл, но мы опасаемся, что при неудачном стечении обстоятельств на ней можно сильно застрять. У Дениса к текущему моменту засланы ответы на первые два теста из десяти.

00:42: У российской сборной пока ещё нули. Из дружественных сборных отметим sancho и Arthur, которые начали с решения простой второй подзадачи в задаче Wiring.

00:31: Таблица как-то заработала. Пока что все баллы не превышают 20, а на первом месте участник из Бангладеша.

00:27: Говорят, задержки вызваны наличием второй площадки проведения. Напомню, что в этом году впервые в истории IOI помимо основной площадки в Тегеране есть альтернативная в Иннополисе в России, на которой пишет сборная Израиля. Видимо, со стартом соревнования на двух площадках возникла какая-то рассинхронизация.

00:18: По состоянию на текущий момент таблица результатов, а также условия задач пока недоступны. Я сейчас выясню, можно ли уже рассказывать о задачах, и попробую пересказать их своими словами.

Всем привет!

С вами на связи из солнечного Тегерана, принимающего 29 международную олимпиаду школьников по программированию, мы с Михаилом Endagorion Тихомировым.

Примерно через четыре часа начнётся первый тур главного школьного события года, на котором Российскую Федерацию представляют четверо классных ребят, представленных на фотографии ниже.

Герои нашего сегодняшнего повествования слева направо по фотографии:
Владимир voidmax Романов, Денис Denisson Шпаковский, Александра demon1999 Дроздова и Егор Egor.Lifar Лифарь.

Стоит отметить, что это чуть одна из самых молодых сборных в истории российских команд: на четверых ребята закончили всего 37 классов школы!

В рамках этого поста мы с Михаилом будем вести трансляцию, в которой попробуем комментировать происходящее на туре, следить за успехи наших ребят и их коллег из других сборных, а также всё то, что покажется нам интересным. Приглашаем всех болеть за участников и делиться мыслями о происходящем на туре в рамках этого поста.

PS Также поделюсь для всех ссылкой на телеграм-канал, в котором мы выкладываем разные фоточки с мероприятий вне туров.

Некоторые полезные ссылки (список будет пополняться):

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7 лет назад, # |
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When will the contest start? :)

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7 лет назад, # |
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Would there be a live scoreboard?

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7 лет назад, # |
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How can we see the live results?

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7 лет назад, # |
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online mirror

1 hour after the original start.

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7 лет назад, # |
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Anyone knows how to submit problems on contest.yandex?

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7 лет назад, # |
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Here you can get some python soft, which watches for submissions of listed participants and sending some alerts on their's submissions. Probably, I fixed all changes in format from 2016 I already found.

PS. To watch for teams other than Russian you should change participants
UPD. Bug in subtask merging fixed

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7 лет назад, # |
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Everything OK with contest.yandex? I got 100 on C, but don't believe it

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    7 лет назад, # ^ |
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    Their checker seems to be wrong because I sent solution which always returns 0 and got 100 points :D

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      7 лет назад, # ^ |
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      As long as I know, they are working together with scientific committee on resolving this issue.

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        7 лет назад, # ^ |
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        Could someone please rejudge all the submissions? My 42pts solution to Problem C should have been a 0pt one. Edit: Fixed now,thanks!

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7 лет назад, # |
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I think Wiring (or at least something very similar) was used in the Moscow Pre Finals ACM workshop this year, though I'm not sure on the original source.

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7 лет назад, # |
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Is everything ok with first problem? I've got 1.82 points for 02.out which is correct.

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7 лет назад, # |
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half of the time has passed
matthew99 : 0
Deemo : 0
Usa team only one participant is on the scoreboard
something wrong ?

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7 лет назад, # |
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OMG geniucos is place 100. That's not fair. Also other chineese participants have very low score. I hope in the last minute to see +100 points and each of this participants.

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7 лет назад, # |
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Women have to cover their head, really? Non-muslims and guests of the country too? In 2017?

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    7 лет назад, # ^ |
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    is this what disturbs you the most in 2017?

    for me what disturbs me the most is that sometimes pornstar is more successful job than programming in 2017

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      7 лет назад, # ^ |
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      For those who don't believe me link1 link2

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        7 лет назад, # ^ |
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        Yeah, credibility of that statement was crucial to the downvotes

        EDIT: Lol, I opened the links and they tell that in fact average programmer makes more, so even your sources contradict yourself xD

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          7 лет назад, # ^ |
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          Now I noticed that link1 provides more than one estimate for yearly income, but at least some estimates mentioned are more than what link2 provides.

          anyway, I believe income of pornstars varies a lot from person to person. what's why I said "sometimes" in my statement.

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      7 лет назад, # ^ |
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      For various definitions of "successful". Money isn't everything.

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        7 лет назад, # ^ |
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        I agree, but what are other factors of success than money? I can think of fame, but I don't think this favors programming over pornstar.

        anyway I don't want to go deep into this discussion since I only wanted to say that women covering their head is not the biggest issue of 2017 and it seems that there are a lot of pornstars fans here who will keep downvoting my comments and furthermore this topic is not related to this blog topic.

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      7 лет назад, # ^ |
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      Oh, but children in Africa are still starving/dying of thirst in 2017 — how can you talk about pornstars!! Is this what disturbs you the most in 2017?!

      Just because you can one-up something bad with the existence of something worse does not make the former any righter. Your argument makes no sense in response of what dalex said.

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    7 лет назад, # ^ |
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    is this what disturbs you the most in 2017? Y ou care about and the fact that your country sponsors the war in the Donbass?

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    7 лет назад, # ^ |
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    We were allowed to take it off during contest, fortunately.

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7 лет назад, # |
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Omg! Attila Gáspár last minute first place submit

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    7 лет назад, # ^ |
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    Tourist strategy maybe? :D

    Though AFAIK he doesn't participate on online judges like CF, so probably haven't heard about it. (He is from the same country as me)

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    7 лет назад, # ^ |
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    :oo

    Like a real boss!

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    7 лет назад, # ^ |
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    Looks like he got WA on the last 13 minutes and then solved the problem just in time.

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7 лет назад, # |
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is it rated?

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7 лет назад, # |
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Train solution
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7 лет назад, # |
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Does anybody have their checker code for A to test the output? I'm getting WA on Yandex, but my checker gives me 85 points.

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    7 лет назад, # ^ |
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    I am having trouble with this too, is there a specific way we are supposed to submit the output files? (Name, extension, etc)

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      7 лет назад, # ^ |
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      You have to submit a .zip file with 01.out, 02.out, 03.out etc. inside it.

      It doesn't have to have all the files, if you submit a zip with only 01.out, it will get judged too.

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7 лет назад, # |
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Really strange results, I think.

So many good rated programmers on CF, do not have so good result on IOI... Both CF and IOI team (other sites too) should think who made mistake ( maybe nobody, but for me something looks wrong ).

What did happen with USA team, I see only one programmer on the list ?

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    7 лет назад, # ^ |
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    These contests are really different from each other. Nobody did anything wrong.

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      7 лет назад, # ^ |
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      but even past gold medallist like reyna and geniucos(silver) did not perform well though

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        7 лет назад, # ^ |
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        Look at the distribution of places among participants from countries doing usually well. China, Iran, Romania, Bulgaria, Poland.

        Has someone seen the logic? Something went really wrong

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          7 лет назад, # ^ |
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          Why is this even a point? Tasks today were slightly different, but still they were normal, algorithmic problems. I'm glad that competitive scene is not boring and other countries have chance to have good scores as well.

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            7 лет назад, # ^ |
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            Normal, algorithmic problems =)

            My point is that this IOI day was very unusual. And pointing about CF / IOI difference — usually there is high correlation between IOI medals and CF ratings. Usually way less good people fail the contest completely.

            Look at the IOI predictions : almost all people predict Reyna and matthew99 to be in the top5.

            You call this "not boring', from a observer's point of view. I would rather name it depressing.

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              7 лет назад, # ^ |
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              Yeah, I agree with you that some results of day 1 are quite surprising, mostly due to the output only task.

              On the other hand, the IOI contest system doesn't check the country of a contestant and give extra points, but it only evaluates the submitted code. If certain countries are so strong, why didn't they submit better codes?

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                7 лет назад, # ^ |
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                Because who would think that on the output only task solutions which are just a simple bfs from a random cell will get > 80 points.

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                  7 лет назад, # ^ |
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                  First problem ruined the standings. Couldn't they give a normal easy problem like in all other iois...

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                  7 лет назад, # ^ |
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                  Not you again complaining about bad distribution of problems topics/difficulty/type/whatever xD

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                  7 лет назад, # ^ |
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                  Why not? lol

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                  7 лет назад, # ^ |
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                  I thought you replied to me and I was ready to start a flame war. =)

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                7 лет назад, # ^ |
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                If Usain Bolt usually runs 100m in 10 seconds we will not run it in 11 during the Olympiad.

                However, such things happen at programming contests. Sometimes contestants just fail completely and nothing can be done about it.

                Similar situation happened this year on Polish OI. Several strong contestants with international medals failed the OI badly. There was a problem with faulty testcases that let many wrong solutions pass. This time it's output only task. You can't be prepared for all, it's more than running 100 metres.

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              7 лет назад, # ^ |
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              In my opinion, in IOI there can be more catastrophe — due to the blindness of the scoreboard, long-duration contest and a few hard tasks, contrary to CF — in which you basically try to solve from first problem. Results depend on whether a student can find relatively easy task and successfully solve it. 3-problems are not enough to prevent the catastrophe, but there is still another contest day.(I still think 6 tasks are not enough, though) You can see what will happen then.

              For predicting, predicting IOI winners are very hard. Predictors take too much weight on ratings on CF — but some students do not even participate in CF, like the winner today, and contest tendency are very different as I mentioned.

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          7 лет назад, # ^ |
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          Yeah, Poland has only two participants scoring in ~ gold medals, terrible performance. Romania claimed 5th and 6th places, terrible as well. However China's scores is indeed a mystery. But it is participants fault not some mysterious underlying factor that went wrong. Output only task may have tweaked results in a bit unpexpected way, but don't put too much blame on it. And you know, you can't expect all strong contestant to excel. You seem to always be very surprised that there exist some contestants that underperformed (last POI as well), but that is just statistics.

          I personally find wiring problem really hard and wouldn't solve it during contest, but train was at most medium to me. Optimal strategy for nowruz in my opinion was to quickly code something that works fairly sane, get ~70 pts and forget about it. And still there is second day and probably some outliers will fix their score.

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          7 лет назад, # ^ |
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          In my opinion that's how IOI should be. The tasks were interesting and none of them required some complicated algorithms or structures — just pure thinking and observations. Yes, the standings are surprising, but how does that make anything 'wrong'

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      7 лет назад, # ^ |
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      I will explain my comment on a little different way :)

      I didn't read tasks, 95% I could not solve something when I see amount of AC. I believe problems had good quality, I saw problemsetters and they are amazing.

      But distribution of tasks today wasn't so good :

      • Difference between gold and silver now 130 points, bronze with 50 points.

      • Many top programmers had really bad place ( as top countries )

      • Big amount of contestants couldn't solve any whole problem, or even half

      • I do not know what is first task, but looks like approximate problem, when other problems are too hard, it is not good to put approximate problem. Top coders will try regular problems first, spent hours and many will have small amount of points, and all other coders will optimize their approximate task 3-4 hours, write fast brute-forces for other and finish as high-placed medalists...

      Best programmer will win at the end, there is no doubt I think, all reds can show second day that they deserved gold, but distribution wasn't good.

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Could someone provide the editorial to today's problems?Thank you!

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Wiring solution (but no idea why)
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    Spoiler
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      7 лет назад, # ^ |
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      For the first transition, how do we store that the left-most point from the next group is already paired?

      In addition, how can we calculate transition 2 with prefix sums?

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        7 лет назад, # ^ |
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        First question: You don't. You just pair it again in step 2. It's fine.

        Second question: What you essentially do is pair B[x..x + k] with R[y..y + k]. WLOG red color is right of black. This means that the sum of lengths is — that is, difference of two subarray sums. You need to be careful when calculating k, x and y, there are lots of +-1 errors to do :-)

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          7 лет назад, # ^ |
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          Thanks!

          Are x and y just the left-most unmatched positions in the monochromatic segments?

          EDIT: Turns out yes, I got AC with your method. Thanks again!

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    7 лет назад, # ^ |
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    Yes, my solution is something like that and it got 100.

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    7 лет назад, # ^ |
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    dunno y but I read "wiring solution" as "wrong solution" ...

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    7 лет назад, # ^ |
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    What about the following sequence of sockets: R R B R B B? I do not think your claim 3) holds.

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      7 лет назад, # ^ |
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      It holds. Connections are 13, 23, 45, 46 and we can partition it into 123 and 456

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        7 лет назад, # ^ |
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        Whoops. So now I'm confused. Which part of your solution isn't right?

        I also tried implementing something very similar and for some reason only got subtasks 2 and 4.

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          7 лет назад, # ^ |
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          My solution got 100 pts xD. All claims I did are correct, but I just don't know why the last one is correct (about optimal point of division moving left).

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            7 лет назад, # ^ |
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            Is the following claim true?

            Assuming claim 4, the optimal splitting point of a series of B R R ... R R B must be (a+b)/2 where a = pos[first red] or pos[blue before first red] and b = pos[last red] or pos[blue after last red]

            Not sure if that's related.

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              7 лет назад, # ^ |
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              Are you trying to consider 4 cases in final version of algorithm or was that a way of stating 4 claims simultaneously :P? I tried similar claims and they work for the subtask with [1, n+m] coordinates and it is even doable to prove it (up to +-1 all versions are equivalent). However for a full version of problem they didn't seem to work. I tried points close to (pos[firstR] + pos[lastR])/2 (+-10 sockets around this point) but it gives WA. Then I gave it another thought and deduced that optimal point lies between (pos[blueBeforeFirstR] + pos[lastR]) / 2 and (pos[firstR] + pos[bluePastLastR]) / 2, but there can be a lot of them and I think we would need to search them all in a solution based on that fact.

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And what's up with Italy? Usually they perform poorly, but today all of them got 100p from B. Looks like cheating to me smh tbh fam.

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And what's up with Italy? Usually they perform poorly, but now all of them got 100p from B. Looks like someone leaked the problems tbh smh fam.

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And what's up with Italy? Usually they perform poorly, but now all of them got 100p from B. Looks like someone leaked the problems tbh smh fam.

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    7 лет назад, # ^ |
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    0 100 0 results are suspicious but there is no way to prove even if they cheated somehow. This competition is based on trusting the leaders.

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      7 лет назад, # ^ |
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      seems to me this is a old problem from years ago link which was used recently. Maybe all of them seems to coincidentally came across it?

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      7 лет назад, # ^ |
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      There's a quarantine as well.

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    There is always a possibility of problems leaking, but I wouldn't suspect cheating based on just that. Maybe they have seen something similar, or talked about techniques needed to solve it. It may look suspicious, but 4 people is too little sample to draw such bold statements. And they didn't score 0 100 0 either.

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    7 лет назад, # ^ |
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    Italy team leaders here. What can I say? We were quite surprised too by our team performance! :)

    We never cheated and never will, for the record.

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      7 лет назад, # ^ |
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      By the way, we are here in Tehran if anyone wants to chat us up about this. We have the same usernames on Telegram.

      I will also add something: one of my research topics is Parity Games (you can check it looking for Giorgio Audrito on Google), so that I even had an already-implemented solution for task Train. I also quickly discussed with Jakub Łącki about that being a known problem, but I didn't raise objections since we never explained that topic to the students.

      If I wanted to cheat, it would have been much easier for me to just pass the Train's solution, instead of working on the Wires' one (which I don't even really know, apart from the hints they provided us).

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        7 лет назад, # ^ |
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        So leaders telling the organizers that their problem is well known isn't enough reason to reject the task? What could be worse?

        Just copy problems from some papers and you have IOI task ready. Here you go future IOI hosts.

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          7 лет назад, # ^ |
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          Well for parity games I don't think it's problematic. But if it's for the other way around (like, that wiring one), then it's quite serious.

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          7 лет назад, # ^ |
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          There are different understanding of an expression "well known". I don't think this was a serious issue, train didn't seem to be known to contestants. However one year ago when I was beta-testing IOI, I solved second subtask of aliens (what could have been treated as full problem for n-1 contestants) by copying my old solution from an old CF E problem (and adjusting it slightly to this problem) it was still not enough for organizers to exchange it. I absolutely hate fact it was posed as IOI problem.

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            7 лет назад, # ^ |
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            Wow, strange to hear such things about the Aliens problem. For me the fact that intermediate subtasks were somehow straightforward DP optimizations is completely discarded by the contribution this problem made to the everyone's knowledge of DP optimization techniques. After that IOI the so-called (at least in Russia) Lagrange optimization provoked lots of discussions, even though it appeared on some Asian judges before.

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              7 лет назад, # ^ |
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              Yeah, I agree that this problem made a considerable contribution into community's knowledge about this very nice technique, however that's not a main goal of a IOI task. There are just three problems a day to make a nice and entertaining contest which is a final destination point of alogorthmical olympiads education for many contestants. They should be given problems that make a fair medal distinction and are very fun to solve, not as Enchom said ones relying on who has read more theory.

              By the way, I suggested a task for that 2016 IOI and it was rejected because idea you need to have to start working on problem could have been encountered on some already existing tasks. I agree, that's a valid point to disregard a task on IOI, but there was a ton of other things you needed to figure out to solve it, it had very nice subtasks substructure and etc. When I heard this I thought "oh, ok, I understand this, that's a fair point", but when I learned that alien task was chosen, which basically asks "Do you know a this do opt technique? If so then 60 ptd. Do you additionaly know this technique nobody knows? Then 100 pts" I felt insulted.

              And by the way, why is it called Lagrange optimization? I have searched through some papers about it, but do not remember seeing Lagrange's name. To me it is called "parametric search"

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                7 лет назад, # ^ |
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                Langrange usually means that you are moving a constraint to the objective function, so it makes sense here. Although I am very surprised that this has appeared in IOI, given that the committee seems somehow strict about restricting the techniques to what appears in the syllabus (from what I have heard).

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          7 лет назад, # ^ |
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          One reason is that there are not many substitute tasks available (because preparing a task requires a lot of work), and the best tasks have already been included in the task set. So if a task is rejected, then the substitute task is probably not so good.

          In fact, I have attended the IOI task selection meeting quite many times, and a proposed set of three tasks has never been rejected!

          Also: team leaders have to translate the tasks, which already takes a long time. If the task set is rejected, then there will be further discussions etc. and it will take a long time before the translation can begin.

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    7 лет назад, # ^ |
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    I don't buy that, why you think some weak people perform well = maybe cheat?

    ishy got bronze in ioi2013 and then he came first in ioi2014, do u think he cheated? ofc not. Plz correct this point of view and attitude.

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My opinions / questions / rants about problems

  • Strong guys performing badly doesn't necessarily mean problemset suck, but it's a great indicator of such. I think that indicator was indeed correct in Day1.
  • Why, why, why did you created that output-only? Honestly, I felt a bit of compassion for the ones who cleared P2 and P3, cause they are literally forced to those boring labor-heavy jobs in one of their most precious moments. Where are all those funs by using brains? Is there some amazing ideas I'm overlooking, or what? I'm very curious for their intention.
  • Yes, P2 is well known (see this), and also one of HYPERHYPERHYPERCUBELOVER's favorite, so I had a lot of opportunity to talk about it. My opinion is this : Isn't it a bit too much adhoc? Yeah, it's a good question, but I think it's quite hard to arrive into the results by standard steps of thought. Maybe some experiences with bracket sequences, maybe some greedy feeling, or... I don't know, some genius aha moment? To prove this, opinion about difficulties varied a lot : Some say it was near unsolvable, some used 4 hours, but I thought that it was not that hard. That's ok, as far as it's not the easiest problem you can give.
  • And I don't understand people copying from OpenCup or recent papers, to make IOI problems
  • Where are all those easy problems? A contest with wiring as only one "solvable" problem? Are you really sure? While there are almost no ties because of that output only (eh), it completely failed to compare their skills, especially for the people who failed to solve P2 (=90% of contestants). I don't know what they were thinking. Do you think bronze medal is not even a medal so it's okay to just randomly give it someone?

In my opinion, this problemset is worser than IOI 2013 Day1, however bit better than IOI 2010 (which was at least experimental and taught setters a lesson)

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    7 лет назад, # ^ |
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    I don't get the hate for output-only problems. They are unique in their own way and I personally really like them. Ever since one was given in 2010 I have been hoping for more. You could get 80+ points with very simple solutions. Maybe there is some labor work if you want to push for a few extra points, but at the least it is a great tie breaker.

    P.S.

    IOI 2010 is my favourite problemset of all time, so perhaps we just have a very different view on IOI

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      7 лет назад, # ^ |
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      Can you tell me what was your very simple solution, and the reason why it works? If it's a very simple solution that's easy to come up, and prove, and get 80 points, then it's hard to understand the scoreboard.

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        7 лет назад, # ^ |
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        My thought-process during the competition:

        • Try something completely randomised. Got WA. Hm, maybe I should give this problem more thought.

        • Okay, so, we essentially want a tree with a lot of leaves. Which trees have a lot of leaves? Well, ones that aren't too deep. I wonder how I can get a tree that's still big, but not deep. Well, the spanning-tree from BFS is as shallow as you can get.

        • Straight up BFS won't give me a tree, though. How can I modify it? Well — just go with the simplest idea — if when processing a vertex in the BFS queue we have already processed more than one of its neighbours, just straight-up throw this vertex out.

        • Yeah, that sounds good. I can imagine it working if I pick some fairly central cell as my BFS origin.

        Got 80~85 with that. My code was essentially just one BFS. I tried picking different origins for it at random, letting it run for a few minutes and ended up with 88 pts.

        There's no need to prove anything — it is super intuitive that this will perform fairly well in the average case and you have all the tests from the beginning of the competition. A quick visual scan of them shows them to be fairly random.

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          7 лет назад, # ^ |
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          I see. So it seems that 80p is not really labor-heavy, but rather about ideas. I apologize for it.

          Still, I don't like the concept of output only tasks. In most cases, after easy hurdles we are usually only left with menial works, overfitting etc. I don't think "trying lot of super intuitive stuffs and submit the best" are that good either. It's not appropriate for 5 hours, and I like problems which I know why my solution work. I don't like to code for all my solving time too.

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            7 лет назад, # ^ |
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            I understand your point of view and I think it just boils down to personal opinion. I have seen more competitors that dislike such problems than competitors who like them, so I can assume that I'm in the minority for the moment.

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          7 лет назад, # ^ |
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          Some people refer to this as "great soluton which can't be more intuitive", so I will just point out that its main motivation "Which trees have a lot of leaves? Well, ones that aren't too deep. I wonder how I can get a tree that's still big, but not deep. Well, the spanning-tree from BFS is as shallow as you can get." is completely shite, that doesn't make any sense if you think about it. However it doesn't change the fact it still does something sensible, BFS will pack vertices densely, will not leave holes that are hard to fill and waste space, but provided justification doesn't hold what breaks "what a great way to think about this problem" argument.

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            7 лет назад, # ^ |
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            What exactly is your problem with my logic? I haven't claimed that this solution is great — I was just showing how you can get good points with a simple thought-process. Maybe I'm just dumb and got points by being lucky with my logic, as seemingly implied by you, but in that case could you enlighten me as to why my logic is complete "shite".

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              7 лет назад, # ^ |
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              Ok, so since I guess my comment was kinda offensive (:f), I better explain myself and I did considerable effort :P.

              First of all, tree has a lot of leaves if it is not deep? I have no idea where have you got this idea. Probably you imagine something like "oh, binary tree has a lot of leaves and trees with long branches don't". But full balanced binary tree has haLf leaves and is shallow but a comb (path with one leaf attached to every vertex of path) has also haLf leaves but its height is half of its size. In my opinion better intuitive description of when tree has a lot of leaves is when every non-leaf has many leaves attached to it on average (which is kinda equivalent, but maybe a bit more intuitive wording).

              However we should not forget that trees on grid are far from general trees. For simplicity assume we talk about trees on empty nxn grid. They have quadratic expansion (|vertices on height d| = O(d^2)) as apposed to binary ones that has exponential. In fact every tree on grid has O(n) height which is already far from shallow.

              I prepared pictures of few patterns we can come up with and measured their performance and their properties.

              As you can see, whether tree is deep has 0 correlation with whether it has many leaves. Diameter of a tree is rather a "macrostructure" property whereas whether tree on grid has many leaves is rather "microstructure" property. It is obvious that BFS will not produce trees 1, 3, 5, but at least for me it is not intuitive which of trees 2, 4, 6 your BFS can produce and I think it may depend on details of implementation and results they give vary a lot.

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                7 лет назад, # ^ |
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                Tree from BFS. Much better than your patterns =)

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                  7 лет назад, # ^ |
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                  I don't think so. This looks like it has density of leaves 1/4 (I am not 100% sure, though) whereas in my patterns densities are 1/3, 1/3, 0, 0, 1/4, 1/4 (it can be proven that 1/3 is optimal density)

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                  7 лет назад, # ^ |
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                  I think its density is also 1/3.

                  upd. I tested a bit, and if I was not mistaken, it makes 0.3 density on large empty field. But with wrong start position, it left some corners not filled at all, reducing density to 0.25.

                  So it seems to be really a good solution, because it produces a good answer, is really simple and needs no handmade patterns.

                  Your patterns 1 and 2 are also good, but it's hard to adapt them to a field with obstacles.

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                  7 лет назад, # ^ |
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                  I agree that this looks like a really good solution and Enchom's 88/100 prove this. I also agree that my patterns are hard to adapt to field with obstacles and that's why I coded substantially different solution which was decent, but not very good also (74 pts). But my point was not proving that this solution is poor. My point was to prove that Enchom's thought process was completely off and he got lucky because his solution performs well because of substantially different reasons than ones he mentioned.

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                  7 лет назад, # ^ |
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                  You gave some counterexamples with fixed patterns, but his words "Which trees have a lot of leaves? Well, ones that aren't too deep." are still correct in general case. This can also be rephrased as: "We should branch out as often as possible."

                  I would also like to add, that idea with BFS come up as opposite to DFS, which obviously generates long paths without leaves.

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                  7 лет назад, # ^ |
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                  "Which trees have a lot of leaves? Well, ones that aren't too deep." are still correct in general case. This can also be rephrased as: "We should branch out as often as possible."

                  No, these words make no sense and no, they can't be rephrased this way. And no, I didn't give pathological counterexamples, these are perfectly fine examples. I see no connection between diameter and number of leaves. However your rephrasing seems fine to me since number of leaves can be expressed using average degree of internal vertices and size of tree, but that is another irrelevant point.

                  Don't defend this point simply because you're on the same side of "existence of output-only tasks" argument.

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                  7 лет назад, # ^ |
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                  Dude, all I mean by that sentence is that if you're gonna be picking some fairly random tree and you want a lot of leaves, by making it not deep you essentially guarantee that there will be a good amount of leaves.

                  Of-fucking-course that you can find examples of deep trees with many leaves and not-deep trees with few leaves. That's the whole point of output-only and marathon problems — nobody is optimising the worst-case scenario.

                  As MrDindows said, in this particular problem the idea of not-deep tree comes mainly from opposing DFS and BFS. Try and run a DFS and check out how many leaves that will give you. My argument about depth essentially just tells you that BFS will be much better.

                  You can continue claiming that my logic was faulty and I was lucky. I am not sure if you're doing it simply because you dislike these types of problems or because you just invested too much time in this argument, but my logic on this particular problem is not off. My statement clearly is not correct in the general case, on hand-made examples, but in this problem it's more than enough to give you an idea that BFS will perform well (as in opposition to DFS or other types of traversals)

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        7 лет назад, # ^ |
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        The `simple solution' that I had in mind was observe (from either eyeballing the data, or a for loop) that there are almost never 10 rocks consecutively (###########). Using this observation, one can divide the columns into groups of 10 using vertical #s, connect these up at the bottom (stop these vertical dividers at row n — 10), and solve the vertical strips separately. For each strip, one can find the leftmost path going upwards, and then extend it to the right once a while to form leaves.

        I think in a grid without 10 #s in a row, this gets about 10% of the total grids as leaves, which then compared to the values of k given are good enough.

        Of course, picking random location and BFSing is even simpler than this (there is also an iterative leaf adding scheme (permute the cells, turn them into .s as long as things don't break) that also does very well). On the other hand, I view construction as a shorter `proof' that the required values of k aren't too hard.

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          7 лет назад, # ^ |
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          All test cases have k ≥ 0.2 * n * m, so this vertical splitting approach might not fetch more than 50% points. On the other hand, picking 100 random cells and doing a BFS from them gets about 20% of the grid as leaves (80/100) which is more than what I had expected.

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      7 лет назад, # ^ |
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      Yeah, I totally agree. I really enjoy oo-task as well and I think they should appear from time to time at IOI.

      In real life, this model is probably more common.

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        7 лет назад, # ^ |
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        Exactly. You get much more approximation problems than actual fully solvable ones. I enjoy such problems much more than problems that just test how much theory you've read.

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          7 лет назад, # ^ |
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          " problems that just test how much theory you've read." — is this how you see typical problems? :|

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            7 лет назад, # ^ |
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            Clearly not all, but I don't like it when some at IOI give off such vibe. For example last year's problem Aliens is a great problem — but the 100 point solution is based on a very unpopular DP optimisation. I hadn't heard of it before that problem and while one can argue that you can come up with it — I think it's reasonable to say that coming up with it by yourself is very difficult. Yeah, it's not a great example since only one person solved that one for a 100, but it illustrates my point.

            I don't say in any way that I'm right, though. Reading things and learning algorithms/structures/optimisations is part of begin a good competitor and it should be encouraged, but I still prefer problems that you can solve even with minimal prior knowledge — just elegant ideas. I really enjoyed problem Wiring from today, for example.

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    7 лет назад, # ^ |
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    For me, even though it is rather impossible to get 100 pts in nowruz, I would call it the easiest problem today. It was really no rocket science to get something not very clever at >70 pts in which case you can say you "did the problem". Remaining 20 pts was a time consuming battlefield for people at the very top. Even though I am against that type of problems on IOI, saying they require no brain and are only labor-heavy jobs is far from truth.

    And I don't know who proposed wiring problem, but I doubt it was copied from paper. Even if it was, I don't think it's something really bad. Copying it from OpenCup was surely not a possibility since call for IOI tasks closes way before it was conducted. Probably if it had been spotted by someone from ISC it would have been removed, but it seems nobody knew that.

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    7 лет назад, # ^ |
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    I always consider IOI as a quite different thing from other competitions like TC, CF, AtCoder, GCJ, FHC, ICPC, etc. In my opinion, the distance between TC and IOI is even bigger than the distance between TC and IMO.

    Let's say, standard algorithmic competitions are freestyle swimming. Then, TopCoder Marathon Match is breaststroke swimming. IOI is individual medley.

    I don't say it's good or bad — they are simply incomparable and different set of people like them. Obviously, high rated people on CF are not necessarily good at IOI.

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    7 лет назад, # ^ |
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    I did the on-site contest, and I scored 100 on P2, never seen anything similar before. Definitely a genius "a-ha!" moment ;-)

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7 лет назад, # |
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IOI sucks since Richard Forster is gone(after IOI2014)

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    7 лет назад, # ^ |
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    Someone correct me if I'm wrong, but doesn't the IOI President have mostly organisational work? The scientific committee chooses the problemset.

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7 лет назад, # |
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So they gave a top-5 task, a top-40 task and the rest can go and play a lottery of "let's see if my ideas/approaches are actually worth anything after implementing them". Nice.

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    7 лет назад, # ^ |
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    Do you mean that usually tasks like today's first is a lottery?

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      7 лет назад, # ^ |
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      Marathon-like tasks in IOI setting – yes. Why are TopCoder marathons multiple weeks long? Because it gives you time to actually analyze and optimize based on your results, try different approaches etc.

      What can you do in 5 hours, when there are two other problems as well? Try to come up with one/two ideas, implement them and pray that they are the right ones, because you really have no way of knowing it for sure. Sure, in this particular instance there actually was a somewhat sensible BFS approach, but I can bet there were contestants with different sensible approaches scoring way less.

      And then there are optimizations. Again, all you can really do is tweak a couple of things, randomize a couple of things and submit your best result. Depending on your luck this can give quite a significant point difference even with the similar approach. The scale of the issue depends on how much can it affect the leaderboard. The score on the last two tasks around medal cutoff is around 30. So if we just look at the first day, this is the task the decides bronze medals.

      Dynamic programming in wiring gives you 7 points. Seven! This is usually reserved for complete brute forces. But at least you can get more than that by picking a better seed in the first task. So in my opinion ISC completely failed to create a contest for all of the contestant and heavily focused on the top 15% and threw a marathon-like task to differentiate the rest. I'm hoping that they realize that a lot of people care about simply getting a bronze medal and give them a fair way to obtain those.

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        7 лет назад, # ^ |
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        Last year we got a really hard prbolem when 34 points were rewarded for obvious exponentional solution xD. Btw I solved wiring problem but have no idea how to produce a solution that gives 7 points and no more. Actually 30 points in total can be scored by O(n^2) approach, I thought it could have been rewarded more, because it really already demanded some careful problem analysis.

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          7 лет назад, # ^ |
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          Isn't your DP solution this :

          Spoiler

          I don't think making this to pass Subtask 3 is easy (at least, I don't see it right now)

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            7 лет назад, # ^ |
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            Ahh, indeed :). When I said about O(n2) approach I actually had on my mind one from point 4) from my solution which in fact works in O(n * s), where s is the size of the largest group of consecutive sockets (which already has subtask 2 as its subroutine), but yours is "tru n^2" :P.

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          7 лет назад, # ^ |
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          It's like really easy to come with a dp solution in o(n^2)(which cannot be upgraded, unfortunately), if you do dp[number of reds matched][number of blues matched]. I came up pretty fasy with that, but it leads nowhere.

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        7 лет назад, # ^ |
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        Dynamic programming in wiring gives you 7 points.

        I just have found out (after stress-testing different solutions) that this subtasks doesn't contain tests with 32-bit integer overflow. So, at least you can be sure that the answer in each test of this subtask is <=INT_MAX =)

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7 лет назад, # |
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welcome to iran, i wish you the best, good luck ! ;)

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7 лет назад, # |
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Do you know if there's a mirror for day 2 too Zlobober?

Update : It seems there is.

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    7 лет назад, # ^ |
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    How do you know, there is?

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      7 лет назад, # ^ |
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      We plan to start IOI 2017 online mirror round exactly one hour after the start of official contest.

      From Yandex website. I think they mean both rounds as they didn't mentioned about only the first day.

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        7 лет назад, # ^ |
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        I knew that, I just thought you know something more :)

        But well, in "Contest links will be available here." they mention linkS, so probably you're right.

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7 лет назад, # |
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