realhype and I are glad to invite you to Codeforces Round #669 (Div. 2), which will take place on Sep/08/2020 17:35 (Moscow time). **The round will be rated for all the participants with rating strictly less than 2100.**

The problems were created and prepared by realhype and i.e. We would also like to thank everyone who was involved in the round preparation:

antontrygubO_o for round coordination and high sense of taste that made our contest IDEAl.

xiryss, Tlatoani, hugopm, Alireza_Keshavarz, jouIej, jo_ulej, dick, Ari, AmShZ, AliceWalkingOutOfHerLife, Devil, morzer, baby_baby_baby_ooh, PM11, postscript, Programmer, saurabhshadow, _overrated_, LordVoldebug, LordVoIdebug, DrNykterstein for high-quality testing.

MikeMirzayanov for Polygon and Codeforces platforms.

You will have 2 hours to solve 5 tasks. One of them is interactive, you should know how to deal with them.

Special thanks to my amazing minipig Schweine, who inspired us during the entire preparation time.

**oink**

Scoring will be announced shortly before the round.

**UPD1:** There will be no pictures of minipig in the statements. :)

**UPD2:** The scoring is **500-1000-1500-2000-2500**.

**UPD3:** EdItOrIaL.

**UPD4:** Congrats to winners!

Div1 + Div2:

Div2:

Auto comment: topic has been updated by i.e (previous revision, new revision, compare).Hats off to you@ antontrygubO_owhile(true) laugh();

epic programmer moment

You can downvote me only if you can continuosly see the given picture of pig for two minutes.

Bruh , Thanks for hiding the pig in the spoilers , that pig was giving me nightmares

Thank you , or in japanese ,

Ari gato , hoping to become a good ProgrammerHey, I understand that you want to make a point, but don't you think tagging is unnecessary? I believe noone likes to be tagged just to see themselves being used for a pun.

Very much Scary like the minipig, huh??????

Ari just told you not to do that

Haha

WeCanDoIt

As a tester I would say that, the contest is really well prepared and the questions are awesome.

antontrygubO_o orz.

He is hero of Codeforces

you forgot the "as a tester" part... could have got you loads more contribution xD

Done. :)

;)

and yes, antontrygubO_o orz

So let me get this straight ,

This contest was tested by Dick , I hope we don't get F@cked

Why is LordVoldebug mentioned as a tester twice?

Tested it twice for "high quality testing".

that's different users:)

one user with two different handles or are they actually two users?

LordVoIdebug & LordVoldebug, i think the difference lies in I & l.

Yaa, becoz of lowercase letter L and uppercase letter I looks lies same

Maybe it's a bug. It's in his username after all.

i have a night now i scared about picture))))) thanks)

My EYES, My EYES !!!

Why is Putin2024 a tag?

Because 2024 is a year, when Russia will choose president. Putin is Putin u know

putin2036

No such pics in problem statements plz

*pigs

To be honest, I was scared by the minipig Schweine when I opened the website. Will he be the hero of the problems? I'm looking forward to it(I love interesting background)!

what a terrible day to have eyes.

istg if you look straight into the eyes of the

amazing minipig Schweineyou'll have goosebumps.I peed myself looking at that

Is this pig the protagonist of the stories in the oncoming round?

I hope we don't have a picture of the minipig in every problem statement.

My new sleep paralysis demon

Yeah, but no matter what, this minipig can't be any more demonic than the

LEGENDARY PONIESTruth!

[deleted]

Its a digon my friend, not a monogon

ok learnt something new today

Opening the site and this minipig is the first thing that comes out to eyes lol. Really looking forward to see him as the theme hero if turns to be so!

That picture is terrifying,please no such pictures in problem statements.

minipig

Oh god! That's terrifying!! Please delete this.

I think, this is better

Can you please put your amazing minipig under the spoiler?

Why people are being dumb and stupid, and overreacting to the picture??

lesson learnt, i wont post shit again, sorry

lesson learnt, wont post shit like that again

laugh is helpful for human,and they trying to make smile in our face. please appreciate them. everybody loves meme.

Good ideas of problems you've prepared! Each is worth tasting.

A very cute pig =))Get a good night's sleep after look the amazing minipig #_# 1h a.m here

Can someone please explain to me why my friend who had rating 1479 before contest got +37 after achieving 3025 rank, and I only got +10 since I had lower rating (1470) than him, and got higher rank than him(2744). This thing is not letting me sleep. Please respond if anyone knows about this ??

Maybe, this post can help you.

If your friend new on codeforces, then his principle of rate-counting may vary with your for some rounds.

Yes this was his 5th contest, and mine 15th.

That because the predictor not computes your result right away. 2 minutes per time may be.

I don't understand. Can you please explain?

There is an extension for browsers called cf predictor. It is useful to know your approximate rating change before the actual rating change. NeiH thought that you're talking about its results.

But still I didn't get why I have less rating change then my friend?

I think it is only because it is his 5th contest. Same happened to me and my senior (although on Atcoder). If both of y'all had 15+ contests then i'm pretty sure he'd get a lower delta than you.

Oh now it's clear. Thank you all.

MikeMirzayanov I think this pic is too much...

pic that is too much is already deleted

i.e's comeback after their drop to specialist is my motivation.

Can somebody tell me what is an interactive problem?

those problems in which the code interacts with the in real time. When we develop a solution for an Interactive Problem then the input data given to our solution may not be predetermined but is built for that problem specifically. The solution performs a series of exchange of data with the judge and at the end of the conversation the judge decides whether our solution was correct or not.

Oh, Now I get the picture a bit thanks for the info

You may look here............

This link was in announcement section....

that was a great help thanks XD

I was going to participate in the contest, but now I've seen that pig I'll better lock myself in my bathroom

Bathroom quarantine

It will be another nice interactive problem :))

Thanks for Reference of

`how to deal with interactive problem`

to learn more about interactive problem see the question of long contest in codechef that always have 1 interactive problem.

We want to know.

Schweine

If you know, you know. :)

antontrygubO_o Hardworking guy spotted

Very magnificent minipig (firstly, i thought it is grey shrek)! I'm looking forward to own resembling one with great pleasure.

scary pig

As a minipig, give me contribution

wait what?

the contest is really well prepared and the questions are awesome. :)

The pig makes me want to skip the contest...

You expect us to solve questions made by so many talented people after seeing that picture. lmao

Good luck!

![ ]()

as coautor and creator of deleted offensive meme I want dislikes and harassment charges on twitter

"UPD1: There will be no pictures of minipig in the statements. :)"

So sad. I think I'll have to skip the round then.

I hope I would become expert in this contest :)!!

same bro

That pig looks hideous.

This picture is so scary！

I was so scared！！

MikeMirzayanov This is too much.Please delete this pic.

Sorry for off-topic but why hasn't ratings for the last round been rendered back again yet?

PS: Ratings are back now

No scary pic in problem statements plz.ToT

Chill dude, check UPD1 in the announcement

Why is this picture here??

i fear the demons that possess the problemsetters.

This pig is causing PTSD.

The picture should be removed imo. Maybe it's some kind of meme and maybe it's funny for some people, but I don't get the picture of the pig. I might be overreacting, but it's extremely disturbing to have to see it every time I scroll through codeforces home page. After all, this isn't creepypasta.

I was not sure about registering for the contest but after the pig appearing in my dreams for two days straight I am glad to say I'll be losing my score today :/

Thanks for putting the pig picture in Spoiler, that's so much better!

`True`

Oh, thanks for delete the pic.

You should have also added,

"Resemblance to any living or dead person is purely coincidental."Unless I am mistaken in identifying.If minipig scared you, you should calm yourself with this video:

if I manage to get AC in C, it would be very nice :)

I hope interactive problem will be nice :) and good luck everyone for the contest.

After so many ups and downs i am hoping that my rating increases. :(

I made the mistake of clicking on the spoiler 10 mins before the contest. Now, I just have to hope that it won't haunt me during the contest :/

Ok this contest is really hard for me! Probably next contest :P

First problem is literally laughing at me

Is it just me who is feeling the questions to be more difficult than usual?

Maybe because this contest has only 5 problems. So problem A is harder than usual problem A but problem D,E is easier than usual E,F.

I think we all overthinked problem A.

When I start practising 1500 rated problems then I'm unable to A,B problems in contests, When I practice A,B level problems then I'm unable to do 1500 or more rated problems. Any solution anyone? :/

Practice both.

Unclear question 1407B - Big Vova , I couldn't understand what lexicographically maximal is.

In the clarification, it says "a is a prefix of b, but a≠b" I don't know when a is called a prefix of b. It would be great if Authors could give example to make things clear. Long and unclear question.

will you please check my submission 92286136? I can't figure out what's wrong.

Try this test:

$$$a$$$ is a prefix of $$$b$$$ if you can remove 1 or more digits from $$$b$$$ starting from the end and get $$$a$$$

Is D DP?

I don't think it can be solved with DP because you can also move backwards.It is a simple BFS the hard part is constructing the graph.

It's given in the question that we can't move backwards

Yeah you are right, I did not catch that so because of that both DP and the Graph approach work I guess.

i solved it using dp and monotonic stack + a BST for transition

Thanks I kept thinking but couldn't implement in time!

Yes see this for O(N)

Even if solution of A is $$$O(n)$$$ why constrain is so low?

Cause problem name

`Ahahahahahahahaha`

I love so much when I understand the exact task just 5 min before the end

How to do problem C?

Keep the index that corresponds to the highest value among the numbers you looked upto now. Send two queries "? i biggestIdx" and "? biggestIdx i". If the result of first query is smaller than the second, you should update the biggestIdx and before doing that you should update A(biggestIdx) with the result of second query. Otherwise, update A(i) with the result of first query. Finally, you know that A(biggestIdx) is equal to N.

Alternatively, it's possible to start queries from index i=1 and index j=n. If p[i] > p[j], do i+=1 and otherwise do j-=1. Don't have to keep track of the largest index this way.

if a % b > b % a, then a % b equals a, otherwise b % a equals b. it also tells you which one is bigger. any time you ask you save the one you got and compare the next you don't know with the one you are left. so you can do this n — 1 times, the position you are left corresponds to the number n.

if you pick two distinct numbers a and b, and ask a mod b and b mod a,you can find the smaller of the two numbers.keep repeating this until you find n-1 numbers and the final unknown value would be n;

Why only N will be the remaining value?

because you always find the smaller number when you compare both the mods,and N is the largest number in the permutation.

Ohhhh... Nice observation... I got all the idea but was not able to observe that N would be the last remaining number.... Did a O(N) loop to find out :)

Similar problem to today's problem E: https://dmoj.ca/problem/ioi11p4

Should have not attempted this contest....Bad Day :(

`HAHAHA`

, this round was`cursed`

.You've nothing to lose.

Personally this is the hardest contest I ever had.

Can some one tell how to solve A and B after the contest?

For A if the number of zeroes is >=n/2 then just type out n/2 zeroes.Otherwise it means that the number of 1 is greater by at least 2 than the number of zeroes so you write out n/2 ones or n/2+1 ones depending which of those is divisible by 2.

B is just brute force and I have no idea how to solve A.

In B just take the largest value greedily to make sure the first GCD is largest and mark it as taken. Out of all the non-taken elements choose one that gives the largest GCD, but in order to not recalculate GCD every time, keep the prefix GCD in one variable and update in each iteration.

In A,

You were supposed to consider as making it all 1's or all 0's because if there are 1's and 0's one of them would dominate the other so the answer would be those max 1's/0's but consider the fact that u cannot take out odd from them so if the max is odd subtract one from them.

Also u might need to think a little extra for the equality condition in terms of the number you want to output

Problem A is very nice for its position. Well done.

If constrain for n were of order $$$10^{5}$$$ then would be more nicer.

i smell the sarcasm here and totally agree

I guess sarcasm is hard to convey in a comment. A is truly one of the better problems I've seen at its position because I had to think for more than 15 seconds to solve it.

SPEECHLESS. You'll become a Master soon. thumbs up bb

Smn plz check my solution on B https://codeforces.com/contest/1407/submission/92280369 I don't know why it fails on the second pretest

The interactive one was nice...

Can help check my submission https://codeforces.com/submissions/williamryank It says it doesnt follow any ? or !

debugging interactive problems seems to be tough. Possibly you had more than 2n queries or something else.

Nice Problems

Always being a tough too solve a interactive problem

Problem A killed me, 3 WA and 22 minutes to get it passed. It was my mistake thought :\

Someone has hints for D?

There are only $$$O(N)$$$ possible transitions, find them by placing building in increasing/decreasing order then the DP/BFS is trivial.

Shouldn't there be C(n,2) transition while it's only possible to move forward?

No, place the buildings in increasing order and consider the closest building to the left and to the right. It can be proven that every transition will be found by this constructive algorithm. So there are clearly at most $$$4N$$$ transitions.

If you have for example 4 2 2 2 2 4 4 4 4 4 4 4 2 it is faster if you move backwards, you can reach the end in 3 steps.

No, as much as I have understood the problem for an input like

`4 4 4 4`

the answer must be 3. You can only jump between i & j if and only if the middle elements are strictly greater or smaller.I kept two stacks where one of them shows the indices of increasing sequence and the other showing decreasing sequence. Then, made binary searches over these stacks and used Segment Tree to get best option among them. However, I got WA at the 5th case and I don't know why...

I used two monotonic stacks to record the indices of increasing sequence and the indices of decreasing sequence, respectively. Iterate height from n-1 to 0.

The same idea as ur, but WA on the 5th case...

See this

For DP in O(N) with increasing — decreasing sequences.

problem A is too hard

Anyone willing to help my submission for div2 qns C. Did the flush and follow query format but still cant pass. Thanks

I think you messed up with indices when assigning arr[idx1] = smth. That should be arr[idx1-1] right??

Idk but that's what ig is wrong as I hv the same soln in C++.

what the A

Hmm.. Now, I'm interested to know how Mr. Kinky Pig inspired this conspira- um.. contest.

Nice problemset btw.

Round gave me PTSD.

Took 36 minutes for A, 8 minutes for B. Life's weird.

Suffering from success xD

kinda same with me, 28 mins for A and 11 minutes for B.

Story of this contest:

Has anyone solved D with $$$O(n * log(n) ^ {2})$$$ solution. I got TLE.

Can you tell me your idea. I tried to solve using binary search and segment tree, my solution complexity is also $$$O(n*log(n)^2)$$$ but got not getting correct answer.

Actually I thought like $$$dp[i]$$$ is the minimum steps required to reach $$$ith$$$ index. Now there are cases if $$$h[i] > h[i - 1]$$$, $$$h[i] < h[i - 1]$$$, or $$$h[i] = h[i - 1]$$$.

$$$ Let$$$ $$$dp[i] = 1 + dp[i - 1]$$$

if $$$h[i] = h[i - 1]$$$, then do nothing

$$$if(h[i] > h[i - 1])$$$ then, find the first index $$$i$$$, let's say $$$j$$$ such that $$$h[j] >= h[i]$$$, and also find index $$$k < j$$$, such that $$$h[k] < h[j]$$$.

$$$if(h[j] == h[i])$$$ then $$$dp[i] = min(dp[i], 1 + dp[j])$$$

$$$else$$$ $$$dp[i] = min(dp[i], 1 + min(dp[k + 1], dp[k + 2], ..., dp[j]))$$$

similary handle for $$$h[i] < h[i - 1]$$$.

I think this is correct but IDK why I got TLE.

see this for O(N)

Is it just me or the implementation for Problem B was very hard?

B was easier then A xD

Well my code for B was very hard for me to implement what trolled me on A was the fact that I always output n/2 even when I had n/2+1 elements.

Actually, it used one of the standard ideas (counting number of 1s and 0s), I am still confused with so many people including myself not getting the idea...

https://github.com/actium/cf/blob/master/1400/00/1407b.cpp seems pretty simple (maybe even easier than problem A)

https://www.youtube.com/playlist?list=PLBqHLq3IFiRLBB96TUEoxBvP80s73uSPo

Do visit the editorials of all the problems given .

I'm really sad I got C solution idea but couldn't code it in time :(

A is cool, but cursed

Pupil to Specialist...Great Contest.

https://www.youtube.com/channel/UCBStHvqSDEF751f0CWd3-Pg/

Subscribe to this channel for all editorial videos .

LOL, What a contest xD

Am I the only one who thought problem A required an output of even size... My mistake ofc but just wanna know if I'm the only dumb here lol....

Yeah A was cursed.

Almost fell for that

Today's div2A problem was the most perfect div2A problem I have ever seen in any CF div2 round.

It would have been better with the test : 8 0 0 0 1 1 1 1 1

Why?

The answer is just $$$1 \ 1 \ 1 \ 1$$$.

to catch stupid solutions like : vector(nbOfOnes, 1) ...

My rating after me solving A after fourth attempt:

You are not alone, for betterment of my mental health and physical safety of my laptop i headed out of the room after first WA.

In today problem C I have submitted O(n) solution in pypy3 it got TLE in tc 6 . Again I have submitted almost same solution and got an AC in 997 ms . I have seen most of the pypy3 codes getting execution time above 900 ms .

But then I have coded it in c++ as it may get TLE again in the systests and got AC in around 300 ms. This is not only today that I have faced this kind of problem , I generally switch to C++ in case of any problem involving recursion . But how may I know that an O(n) can also get TLE .

So to all the respected problem setters please check if the python solution is accepted with a considerable margin or just increase the time limit.The issue with python is that it never take a fixed amount of time for running a piece of code, running the same code multiple time can sometimes yield different results. This is not just on codeforces but many other platforms also faces criticism for uneven time limits of python but you see its really not in their hands because increasing time limit can make unintended brute force solutions pass the system tests. You can correct me if I'm wrong.

I am not blaming them . I just want to say they should check if the python solution is getting accepted with a considerable margin . If not they can just increase the time limit by some milisecond . 1 to 1.5 or something like that.

They clearly can set TL by language which they dont

Problem with python programmers (who do CP) is they only know how to whine about time limit. First of all, you use

not the besttool for a task(python instead of c++/java) then you complain about things not being same for you, like WHAT?Also there are many high rated python coders, so clearly things haven't really been unfair,

youjust don't know python enough for CP. Either learn python more indepth, understand all intricate and subtle things about it then use it for CP (you won't need to complain then) OR use a more appropriate tool(C++/Java/Rust/..)I agree with you that this interactive had kind of a tight time limit.

It is possible to pass just using the default PyPy IO 92228274 (951 ms), but the TL is tight. Important to note is that the built in IO in PyPy is far slower than CPython IO. The same code in CPython runs in 655 ms 92292022. The work around for PyPy is to use a drop-in fast IO template 92308464 (670 ms).

About your code 92261279, there are two ways you could improve it.

Don't import a crap ton of stuff that you don't need/use. Your unused imports at the top take like 150 ms in PyPy3. Also by doing

`from math import *`

you are overwriting the very useful built in pow function.You can make use of fast IO templates like this to greatly speed up the IO in PyPy.

With both of these improvements your TLE code runs in 748 ms in PyPy 92307825.

As a final remark, it is possible to get below 300 ms even in PyPy 92288195.

Edit: case was wrong nvm

I didn't understand why (4->5) is incorrect. I think the answer will be $$$1$$$ for this test case.

Oops apparently I was too salty to think of cases properly >.>

so 4->5 should be feasible

In question A div 2 We can not remove consecutive elements so for case 4 1 1 0 0 Why answer's (1 1) and (0 0) are getting passed , Since in both cases consecutive elements are getting removed.

Statement says "The elements that you remove

don't have to beconsecutive."They might or might not.

ok

So this question basically tests our English skills?

This was my first interactive problem which i had tried and got wa only for using long long variable..Is there any restriction about using long long at interactive problem???

Sorry i don't know about that matter.

Thank you kindly for any good response. @realhype

Nope. I use long long everywhere and got AC in C

it is saying that y is out of range because your program is asking x == y and you cant do that

`"? x y" (1≤x,y≤n,x≠y).`

Nope not for that.I was so foolish and i used %d while i was using long long. Later i fixed that and got ac for both int and long long.

Maybe i tried the wrong program anyway , good job

Who could tell me the correct algorithm to solve Problem.D ? Thanks!

What to do if one dosen't get the logic, like me in problem A.?

Skip the problem.Do the others and comeback.Each person can probably solve questions upto slightly above their rating. By that logic I usually try up until D, many a times skipping questions I didn’t get in 10 min.Today is a good example. Didn’t get A in 10 min, so skipped it, did B and C, came back to it, it worked out.

In div2C, I got system test

failed, submitted in C++17 and now even sample test is not passing for that solution(if it was shown during contest, I could have tried to correct it). While when I submitted the same code after contest in C++11, it gotAccepted. Why is this weird behaviour? i.e, realhypeLink for WA(C++17)

Link for AC(C++11)

https://en.cppreference.com/w/cpp/container/set/erase — "References and iterators to the erased elements are invalidated"

After you do

`st.erase(a)`

, the memory location that`a`

points to is no longer valid. So`cout << (*a)`

can print non-sense. Get the value before erasingOk I got your point, but then why same code in C++11 is giving AC. It should also give WA.

undefined behaviour. Guess you got lucky cuz the old compiler decided not to overwrite the freed memory

I don't know good or bad but why was Problem A laughing at me :sob:.

Good problems on math.

Bye, my rating..Even if we know each other for less than a day, I still miss you from time to time.。

Hello everyone,

I received a strange runtime error on TC 10 in today's Problem B — "Big Vova" in pypy2.

Here's the link to simplified code, 92298672

But after adding a useless line to the solution it got accepted. Which was:

Link to the accepted solution, 92298831

If you read my code, the above snippet never gets executed according to my logic.

I am confused if this is pypy2 fault or cf. There were few other python submissions that had the same problem like this one for example 92244789. If anyone of you knows what's happening please do let me know.

Link to my original submission during the contest 92248591

I would like i.e realhype to see the submissions. Thanks in advance!

What you have there is a bug in PyPy. Me and some friends over on discord have been able recreate the segfaults locally in both PyPy2 and PyPy3, and it has segfaulted on every machine and version of PyPy we've tested it on so far. I've never seen anything like this before.

Will definitely make a bug report once we find a minimum working example.

Here is my bug report https://foss.heptapod.net/pypy/pypy/-/issues/3297

raj1307 The bug is now reported as fixed.

Well, thanks for that :)

So,when will move the cheater and change the rating?

what is wrong in my code ??

https://codeforces.com/contest/1407/submission/92301842

#define endl '\n'can i get input and output files for q1 and q2 because i am not able to find my logic anywhere wrong for q1 and in q2 when i checked for wa for input2 it is showing same same as answer and also check my code for q1(https://codeforces.com/contest/1407/submission/92244915) and q2(https://codeforces.com/contest/1407/submission/92258924)

got the mistake for q1 but still not for q2

In B(q2) you are sorting based on best gcd value with only the maximum element this won't work since we need gcd of all elements upto i for each value of ci.

Instead you need to keep choosing the element that gives best gcd value with gcd(all chosen elements till now). And initially choose only the maximum element.

thanks got it

Question D. Discrete Centrifugal JumpsWith explanation.I hope my comments will help others to underastand the solution :)Nice

Why I was the last but still give me points?

This is caused by the way new accounts have a "special" calculation of rating. After like 5th contest that effect should be negligible.

ohhhh!I have known 10 minutes ago,thanks a lot.

in problem D what is the problem if I get the minimum of going to {current index + 1, the first number bigger than me, the first number lower than me, the biggest number less than me, the lowest number bigger than me} using dynamic programming???? my code

Doubt in DIV2 — A

How can we directly remove all the 0's and 1's as it is written in the problem statement that "The elements that you remove don't have to be consecutive." So, if the input array is 0 1 0 1 1 0, then according to the editorial the output must be 0 0 0. So, we have removed two consecutive 1's (i.e. 1 at 4th and 5th position). This is against the problem statement. The correct output for the array 0 1 0 1 1 0 should be 1 1 0 (2nd 4th and 6th position)

Please explain. Thanks in advance :)

I guess it meant the elements we remove don't have to be consecutive ie it's not necessary for them to be consecutive,

but they can be.It is a real english and its meaning is the elements remove cannot be consecutive.

Otherwise it should be cleared that the element may or may not be consecutive. Please review the problem statement.

What I said was actually what the question meant. That's why they're allowing consecutive elements.Maybe the meaning of the statement is not clear for everyone.

92240790 1407A - Ahahahahahahahaha I did pretty much what the question asked for. Coded the brute force approach, but I cant seem to find where I am going wrong ?

What is the idea of that code?

to calculate sum of odd positions and even positions. then checking whether oddSum — evenSum is > 0 or < 0 or == 0. then accordingly removing 1s from either the odd positions or even positions or removing nothing (== 0 case).

If you remove an element, the remaining elements switch group. Previously, all those that are odd indexed after that element become even indexed and vice versa.

Hence it is easier to just keep all the same digits (either all 1's or all 0's).

oh okay thanks!

There is another simple strategy: We can group the input in groups of 3 elements.

In such a group there is allways either at least two 1 or two 0. So we output foreach such group "11" or "00".

The case that length of input==2 must be handled separate.

I think problem A was good retrospectively. But only 45% of the participants manage to solve it. If you look at past problems like 651, 652, 654, more than 100% manage to solve problem A (The number are obviously wrong). Yet why such a big variance on problem A results ?

thanks for nice contest :)

I really enjoyed Problem D, thanks!

Such a nice D and E. Wow.