Ok so now CF will host contests prepared for middle school students. Kill me now!!!!!!

This will hopefully be a good contest for those of us training for olympiads :)

Which problems are going to be based on the olympiad?

But Is It Contributed?

Or Is It Rated?

The start time changed from 16:05 UTC+8 to 21:35 UTC+8... At least I can go to bed before the next day.

mathforces?

round #503 and error #503(unavailable Gateway).. another coincidence??... hopefully..not this time :)

I guess despite the original contest being IOI styled, this round will be CF styled, as always?

Hope to be a good contest as it seems to be, wish you all guys better ratings...

Great to see izban is one of the problemsetters!

Are there any shared problems between both divisions ?

Пересекается с днём физкультурника в Архангельске

Please provide some more tutorial about an interactive problem.

Please...

Everybody jump on the downvote train!

Можно ли участвовать, если ездил в ЛКШ.Июль?

Great round! The time really fits me.

As the prizes of the previous round is postponed, will there be prizes in this round?

,

I hope it won't be unrated like Round #502 (Laugh

this blog full of downvote comments

Why are you people downvoting every single comment? Just for fun? It completely ruins the whole discussion, because if anyone says anything, he will get downvoted to hell.

Пересекается с Codeforces Round #503 (по олимпиаде ЛКШ)

Is it disrated on downforces?

Why most of the comments in this post are hidden?I can't find the reason why you downvote so much.(Because of this I upvoted all the comments)

Why are all comments downvoted?

please delay/starts it 15 min later than scheduled time because there is a contest at atcoder also.. please this is a request.

...

Hi

I made 10 accounts in Codeforces and downvoted some comments here, and of course others saw that and kept downvoting the comments. Some of you may already knew this but my purpose was to show that lots of people here are just dumb followers and they don't have any brain to decide what to do, they just copy others' actions.

So to the guys who got downvoted in this community or as some guy in other blog said are not a clown to make others laugh, don't worry, it's not your issue.

Thank you for being idiots.

Why someone was so bored to downvote everyone?

I have short by the way.

That one person who'd like to comment but too afraid of the downvotes.

If you want to see the comments in this thread without clicking on every link or you have a strong will and always want to see every downvoted comment, then install my brand new super duper userscript created in the last 5 minutes: https://openuserjs.org/scripts/mraron/Codeforces_Show_Bad_Comments (you may need something like tampermonkey addon to get it working though)

[/END of self advertisement]

Hope the description of the problems can be concise.

Scoring?

Anyone join the campaign to turn all comments to upvote (except those ask for downvotes or unhelpful comments) ?

Freaking A >(

problem c is much harder than problem b

How to solve problem D?

How to solve div.2C?

Interesting problems, how to do Div1C?

For div2c the idea is to always select the minimum voting cost until it's time to choose the last voter... is this approach right? and if it is, how to implement the last part of it ?

Div.2 C is bunch of priority queues and sets and maps. I was not able to finish it in time :'(

Does every interactive problem on CF have to be binsearch :P? (OK, I know there are many counterexamples, but number of such problems is too high and they are rarely interesting)

Any idea for Div1 C?

Btw, C is a bit too hard, but today's problemset is good. Problems are balanced between thinking and implementing. I enjoyed B a lot.

can anyone tell me an idea/solution of pb "C. Elections" in recursive + memorizing

My story in A

Unlucky round for me.

I started by quickly solving A, then moved to C. Wrote a solution in 20 minutes, and only then red the second sample. I really need a browser extension to replace "loop" -> "self-loop". Then I moved to b, and got 2 WA's in test 16. What is the problem?

int answer(int i = -1) {
	if (i > 0) i = (i%n+1);
	cout << "! " << i << endl;
	exit(0);
}

I solved the problem zero-indexed, so I had to fix the indexes by adding 1. But if i = 0, it outputs 0 giving WA. I didn't realize this was the mistake, shuffled some code around, and got "pretests passed". Then only later I realize this and had to resubmit.

But yeah, I couldn't solve C, so I don't mind losing rating.

Does anyone know what's the issue causing WA on pretest 5 in div2C?

Hi i thought I found someone violating the rule of codeforces contest

higu and adityasr share the same solution for Problem C of Div 2

This the first time for me to enter a contest , in fact I couldn't solve any problem as I misunderstood many points and also I haven't practice a lot ,but I hope to do better than this the next time.....

See!!

I told you everybody that Benq is going to be the next world conqueror of CP.

rip everybody

Benq wil beat you all!

could any one tell me what is that system testing that have started after finishing the contest,please??

How to solve Div1 C? I passed pretest but I'm not sure my solution...

I think today's problems are very nice.

But,I think the difficulty of solving B and the difficulty of solving C is too different.QAQ

How to solve Div1 C? I passed pretest but I'm not sure my solution...

Is DIV1A/DIV2C same as http://codeforces.me/contest/458/problem/C

For div2 C, first lower down every other party to at most k votes, then we can get enough votes from those who need least without considering if he is from the most voted party. Check every k from 0 to m, we can find the minimum cost.

I pass pretest with this solution in 1900ms, and reducing time to 1100ms using priority_queue.

Hope it won't get a TLE on the system test :p.

Did someone passed Div1D with O(n³)?

In div. 1 C, about 70% submissions that passed the pretests, failed later on test 20. Why wasn't this test added to pretests?

Looks like most of the solutions to Div1 D (1019D - Large Triangle) are with binary search. I didn't come up with the right solution, but managed to squeeze an O(n³) one (41491248) instead, using 1950 milliseconds out of 3000. It involved a trick and a dirty trick. I'd like to elaborate on that, to increase awareness of such tricks for contestants and problemsetters alike.

The trick is just loop unrolling, which is around this part:

enum span = 4;
for (v = j + 1; v + span <= n; v += span) {                    // instantiate 4 identical blocks at a time
	static foreach (r; 0..span) {{                         // so that they can be computed in parallel
		auto t = pjx * p[v + r].y - pjy * p[v + r].x;  // pjx and pjy are p[j].x and p[j].y
		ok |= (t == sp) || (t == sn);                  // t can be positive or negative
	}}
}

The static foreach is expanded at compile time into 4 blocks with r = 0, 1, 2, 3. In C++, one way to do it is to use a #define inside the loop, and instantiate it manually 4 times instead.

The dirty trick is doing all of the above in signed int32: as the program is run in a 32-bit environment, it helps greatly. If we find the triangles which have area equal to s modulo 2³², only then we check again using true but slow int64 multiplication and comparison.

Now, as a problemsetter, you can't really do anything against loop unrolling. In fact, in simpler cases, a good optimizing compiler does that itself under the hood.

Against the using int32 part though, it is possible to construct a test case. Make all coordinates divisible by 2¹⁶, and s divisible by 2³². This way, all multiplications modulo 2³² will result in zeroes, and the actual O(n³) work will be done by the int64 part of the solution. In this case, making it run for five or six seconds.

Feeling so uncomfortable in this round.

Are you planning to change this feature/bug with bomb on sample other than the first one? It cost me a victory...

How to solve Div1D

Nice TOP3! High five Marcin_smu and Radewoosh :))

Btw, it seems that many people tried some heuristics to C and all fo them failed. I have to admit that I also pushed some shitty solution... But it seems it was less shitty than others :P. I basically keep all the time some set of vertices that is an independent set and when I see a vertex that is not reachable in two steps from others I delete all its out-neighbours from set and add itself. And I do it until I run out of time or until I get a solution — whichever is sooner. And it luckily turns out that second case is always sooner :P

I opened problem E after the end of the contest, and found it was actually an extension of my recent CSA problem. (That extension completely changes things, so it's not a copy) Although I don't have any good ideas, I regret myself of not opening E :)

And big thanks to setters for the great round! Problems were really interesting. It looks even better because last round was very poor. Thank you so much for the effort!