Supermagzzz's blog

By Supermagzzz, history, 8 days ago, translation, In English,

Hello, Codeforces!

We have planned this round as a Div. 4, but testing showed us, that problems are more suitable for Div. 3. Therefore, instead of the previously announced Div. 4 we will hold Div. 3 round.

<almost-copy-pasted-part>

Hello! Codeforces Round #644 (Div. 3) will start at May/24/2020 16:35 (Moscow time). You will be offered 8 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have a rating 1600 or higher, can register for the round unofficially. The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 8 problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as trusted participants of the third division, you must:

  • take part in at least two rated rounds (and solve at least one problem in each of them),
  • do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

The problems for this round were invented by MikeMirzayanov and prepared by me and Stepavly

Thanks to MikeMirzayanov for platforms and coordination of our work. Thanks to antontrygubO_o, iankury, bfs.07, kocko, Ilya-bar, arsijo, pashka, Gassa, neal, vovuh for help in round preparation and testing the round.

Good luck!

</almost-copy-pasted-part>

UPD: Editorial is out

 
 
 
 
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8 days ago, # |
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Look forward to this round. Thanks for offering div 3 and 4!

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Finally other author than vovuh for div3 contest

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    8 days ago, # ^ |
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    Last time vovuh wasn't one of the authors of a Div3 contest was 23 Div3 contests ago!!

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      7 days ago, # ^ |
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      As the contest was prepared to be a D4 one, also everyone knows that vovuh doesn't prepare D4 contests(at least until now), so there is no vovuh as an author here.

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      6 days ago, # ^ |
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      I hope that you rock this round. All the best chota haath mota pait.

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    7 days ago, # ^ |
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    Because of the round previously announced as div. 4 :D

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      This made me LOL, at least provide translation if you don't want downvotes lol

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        6 days ago, # ^ |
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        Steiner1411 chill h bro! Ye apne(indian) he hai jinhe samajh me sab aata h par phir bhi downvote karte h! Par apne ko ghanta pharak nhi padta bas thodi hasi thitholi ho jaye wahi bahut hai. I dont know why they cant take things as a joke.

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          6 days ago, # ^ |
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          Yaar, tu to bahut chalu hay

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8 days ago, # |
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Great!

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The time link always shows the Moscow time rather than the local time, please fix it

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    8 days ago, # ^ |
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    By the way thallium is not vegetable its an element . How you guys come up with such unique usernames??

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8 days ago, # |
Rev. 2   Vote: I like it +53 Vote: I do not like it

Div4 was an experiment. Keep maintaining the standard of Codeforces.

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[deleted]

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Open at your own risk
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Thanks for the div3 instead of div4 :)

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This weekend is lit , as CF contest at 7pm IST and at 9.30 Codechef Cookoff . Noice

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    6 days ago, # ^ |
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    Yup. And then 2 codeforces round + lunchtime this week!!

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8 days ago, # |
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MikeMirzayanov Do you have any plan to run Division 4 regularly?

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    8 days ago, # ^ |
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    That was supposed to be div4. So, i think yes, he do.

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[Исключен]

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    8 days ago, # ^ |
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    did you even bother reading the first line of this blog...

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    You are accepted anyway, isn't that great!

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Исключен

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8 days ago, # |
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Where is vovuh :|

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First time there will be 8 problems in Div3!!!

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Rev. 3   Vote: I like it -13 Vote: I do not like it

Regular round for beginners should arrange more frequently...

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Is it rated since Division was changed from 4 to 3?

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    8 days ago, # ^ |
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    yeah, it is rated

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    8 days ago, # ^ |
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    When it was div4 then it was rated for div4. Now it is div3 so it is rated for div3.

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8 days ago, # |
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Every time contest is organised after so many days, I get excited. It's other thing that I'm not able to solve more than 2 or 3 problems in contest.

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When a person sees C#

Programmers
Non programmers
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I'm starting to miss vovuh already. :(

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not div 4 Orz

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8 days ago, # |
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We missing out on a potential DIO meme here xD.

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Can anyone suggest best apps for writing code like codeblocks ( other than codeblock) for a windows user

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Isn't it unfair that those who have ratings 1599 can participate in the contest and make their ratings 1800+ but those with rating 1601 can't change their ratings?

You will be offered 8 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600

I'm sorry but I have found candidate masters who find questions from div 3 contests interesting.

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    8 days ago, # ^ |
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    So,what should be the boundary? If it is x, will not it be unfair to people with x+1 rating? :v

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      8 days ago, # ^ |
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      One solution may be to cap the maximum achievable rating in div3 contests to 1600.

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        7 days ago, # ^ |
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        It is a matter of fact that nearly all reds started at aprox 1580-1595 and then instantly jumped over 2000... where they happily lived ever after. Not.

        What is this nonsense about? You where below 1600 for like two years.

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        7 days ago, # ^ |
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        What about people rated 1599 then? Not unfair to them?

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    7 days ago, # ^ |
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    Codeforces never let someone stay above his skill level. If someone has skill level for rating 1600 then if he somehow gets 1800 rating then he will come back to 1600 within two contest. If someone gets uneven rating increment in one contest then he will distribute it in next contest. He will not take the rating home and sell it in local market. Codeforces will never let him carry a rating which he doesn't deserve. So one uneven increment doesn't matter too much.

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    7 days ago, # ^ |
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    I find a master who thinks div3C is too hard for my level. Yes. He is myself.

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    6 days ago, # ^ |
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    This is a platform for practice so,if u are looking for placements wise even if u are expert company will hire you, if you are skilled. Happy coding!!. Take it in positive.

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_The problems for this round were invented MikeMirzayanov and prepared by me and Stepavly._

Please insert by before Sir MikeMirzayanov

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7 days ago, # |
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Finally a contest after so many days. It will now be rated for me. Excited!

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7 days ago, # |
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7 days ago, # |
Rev. 3   Vote: I like it +1 Vote: I do not like it

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Will there be more Div 4 rounds??

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I miss vovuh. His problem explanation is great. I don't face difficulty understanding his words. Hope for the best for the new writer....

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8 problems!! This will be fun.

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It was a div-4 contest when I registered. suddenly I found it div-3.

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And here we go "A non-vovuh Div3 round" after consecutive 23 Div3 rounds. Hats off to vovuh for this 23 round streak. Good luck to new authors.

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7 days ago, # |
  Vote: I like it +19 Vote: I do not like it

Maybe you could add one or two easier problems and delete the hardest problem, and then the contest could be div3 and div4, just like div1 and div2 in the past.

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7 days ago, # |
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Will the problems include sub-tasks?

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What acctually means "Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes"?? Can any one elaborate this line? please

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    7 days ago, # ^ |
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    There occurs a penalty according to the time you submit a correct solution. The more time you take more penalty occurs. However, if you make a wrong submission then the extra 10 penalty will be imposed per wrong submission. Correct me if I am wrong.

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Can someone elaborate on what this line means:

do not have a point of 1900 or higher in the rating.

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    7 days ago, # ^ |
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    If someone has maximum rating 1900 or more then he will not be considered as trusted participant.

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      Can someone elaborate on what this line means: trusted participan

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        7 days ago, # ^ |
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        Sometimes some high ranked coder make fake account and participate in div3 rounds. So they get high rank on that contest. As a results, real participants can't know their real position in div3 contests. So, people who din't participate in at least two rated contests, then he will not be a trusted participant to stay in final standing. So, there will be no fake account on final standing and real participants can know their real position on standing.

        A high rank coder can loose rating on purpose to reach less than 1600 points and get a good rank officially in div3 rounds. So if a contestant has 1900 or more maximum rating then if loose rating towards less than 1600 them he will also be not considered as trusted participant and will remain out of final standing.

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Thanks 4 the time

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7 days ago, # |
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I am sorry for the dumb question, but what does testing showed us, that problems are more suitable for Div. 3 mean? I mean, how does testing reveal suitability of problems for whatever division?

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    7 days ago, # ^ |
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    same question

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    7 days ago, # ^ |
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    When testing, they make a virtual contest environment and asks the testers to judge the problems. Based on their feedback authors decide.

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      7 days ago, # ^ |
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      By "judge the problems" you mean "assess problem difficulties"?

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        7 days ago, # ^ |
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        Yes and also the clarity in the problem statements, testcases and everything.

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7 days ago, # |
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Good luck to all! Live solving the problems 15 mins after the contest finishes:

https://youtu.be/gFepyRGn1vs

Only on Algopedia!

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7 days ago, # |
  Vote: I like it -6 Vote: I do not like it

can this contest be postponed for tomorrow? The codechef competition is also on the day. for last few days there was no competition on both sites and here we go. We have both on the same day and by half hour gap. The codechef schedule was announced before a long time.

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    6 days ago, # ^ |
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    This round starts one hour earlier than the usual time, so that those two contests do not overlap. 25 minute break between rounds should be enough for you.

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7 days ago, # |
Rev. 3   Vote: I like it +6 Vote: I do not like it

To be honest,Vovuh & MikeMirzayanov always makes nice problemset:)

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odd time

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Eid Mubarak to all the contestants. Hope it will be a great contest for us before EID.

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It is div4, lol.

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[deleted]

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6 days ago, # |
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It should not have been a div3.

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I didn't receive a reminder email

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The comment removed because of Codeforces rules violation
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    6 days ago, # ^ |
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    WHAT?! This is the first time I see that codeforces removes comments

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      From time to time somebody talks about problems while contest.

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ConstructiveForces.

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6 days ago, # |
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Quick editorial in Python. For more editorials visit this discord: http://interview.solutions

Question A
Question A - Tutorial
Question A - Code
Question B
Question B - Tutorial
Question B - Code
Question C
Question C - Tutorial
Question C - Code
Question D
Question D - Tutorial
Question D - Code
Question E
Question E - Tutorial
Question E - Code
Question F
Question F - Tutorial
Question F - Code
Question G
Question G - Tutorial
Question G - Code
Question H
Question H - Tutorial
Question H - Code
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    6 days ago, # ^ |
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    Men, thank you so much, you make this editorial faster than the speed of light

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    Thanks a lot man!! Just looking at your editorial for a few minutes I was able to solve C & D in just a few minutes right after the contest & also realized how easy they were! I unnecessarily made my solutions complex, I should have thought & observed little more:(

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6 days ago, # |
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Problem G better version: https://vjudge.net/problem/LightOJ-1330

Solution available on net.

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1st time I solved 6 questions in a contest. Yipee!!

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Can anyone help me with what peculiar case can be in Case#2 of problem F ?
I kept getting WA on Case#2.

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G is same as lightOj 1330.

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    6 days ago, # ^ |
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    Not the same because you are not asked to output the lexicographically smallest, but is true that the solution of LightOj works for today's G

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6 days ago, # |
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is div3 hard? i couldn't able to solve single problem

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It's the first time I was able to solve ABCDE. I think it's not DIV3, it's between DIV3 and DIV4 or 3.5

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    6 days ago, # ^ |
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    Same goes for me. First time solved 5.

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Why greedy isnt working for G?

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Can someone tell me why this got TLEd - https://codeforces.com/contest/1360/submission/81243429 and this got accepted - https://codeforces.com/contest/1360/submission/81245539

Both are exactly the same code.

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Seriously it's div3. Nope. I think it's the actual div4. Where you would have trivial A, B, C, D, E and not so trivial F, G and really hard H. G was super cool though. Felt like a fool after solving this.

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6 days ago, # |
Rev. 2   Vote: I like it -15 Vote: I do not like it

The input data of Problem H is 6MB at worst. Using cin will get TLE, using scanf will not get data because of strange '\n', using some other library also fail. A problem's difficulty should be how to solve it, not how to input it!!

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    6 days ago, # ^ |
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    Well, actually, you can read non-whitespace character with scanf as such:

    scanf(" %c", &character);

    The space before the %c filters out all whitespace, so it becomes a non-issue. Alternatively, just use getchar/normal scanf and check whether it is a character between 'a' and 'z'.

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    6 days ago, # ^ |
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    I didn't believe there was an issue with problem G, since I solved it using cin, with the same expected complexity as given in the editorial.

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    6 days ago, # ^ |
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    Check the editorial. Our solution use cin and get AC.

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MikeMirzayanov Does increasing the number of questions of DIV4 makes the round DIV3? I hope thats not true for DIV2 round.

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    6 days ago, # ^ |
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    I (personally) don't think so, since usually some contests that are harder or have more problems have their time extended, instead of increasing their division. Also, I (again, personally) believe problems G and H were more suitable to a div. 3 contest, rather than a div. 4 contest.

    Then again, I did not participate in the problem-setting nor testing of this round, so who knows ¯_(ツ)_/¯

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6 days ago, # |
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I actually spent more time debugging than reading and thinking combined..

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Can anybody kindly show me a test case where this bruteforce(!) solution fails?

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I hope this is the first and last div3 that you organised.

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Can I see due to which test case my solution was hacked>?

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Can Someone Explain how to do F?

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Guys head over to CodeChef Cookoff, it'll surely be more worth your time than those simple, time-wasting Div 3 problems https://www.codechef.com/COOK118B

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What is the best time complexity one can achieve for problem F ?

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    6 days ago, # ^ |
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    Definitely this is not the best, but let's analyze this:

    Take i-th string. Then for each position j, you try each of the N letters in the j-th column of input. There are N*N*M possible strings making this procedure. Then you need to check if that string works, in N*M complexity. Final complexity of this is O(N^3*M^2). I wonder if there's a O(N*M) solution

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For the first time in my life solved 5 problem. Thanks to the authors.

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I think this is not the standard Div. 3 Level. Make it Rated upto Div 4 only.

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In F , I used the brute force concept and my run time complexity was O(n^3*m^2). How to solve it with larger constraints ?

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    6 days ago, # ^ |
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    It can be solved in O(26*N*M^2) by trying to change every letter in the first string, to every possible value. Then you check if that string works.

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This div3.5 contest was awesome

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In problem f, why the code is getting RTE 81315889?

can anyone please help me?

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I think it was better to keep it div4. Div 3 E F problems are generaly much harder than all of these problems.

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What is the Hack for problem D?

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6 days ago, # |
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MikeMirzayanov

This two people are copying codes i guess https://codeforces.com/contest/1360/submission/81303569 and https://codeforces.com/contest/1360/submission/81306780 other solutions of them are also similar. Their previous contest submission are also same. Sorry for bad english. Please look into such acts.

Edit: I just saw that all their submissions are similar. They are taking plagiarism to next level.

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6 days ago, # |
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Can anyone help me with what peculiar case can be in Case#2 of problem F ? I kept getting WA on Case#2 https://codeforces.com/contest/1360/submission/81313261

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6 days ago, # |
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I used bitmask dp to solve F in O(t*(1<<n)*m*n^2) and It got accepted in 31ms ... Can anyone provide a testcase which can find the upper bound of runtime .....here is the submission — 81323908

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    6 days ago, # ^ |
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    I did the same but i forgot to mark my visited array because which my solution is running exponentially and guess what.. it still got accepted

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    6 days ago, # ^ |
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    I think this will pass. In the recent educational contest my soln with 3e8 operation got accepted with 900ms.

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      6 days ago, # ^ |
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      Actually I know this will get accepted....My problem is that it got accepted in 31ms which I didn't expect and it's 0(10^8). While O(10^9) shows tle in 5 sec....

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6 days ago, # |
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how did you guys approach, your thought process to find out how to arrange all the one's

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6 days ago, # |
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I used a network flow to solve G lol. Anyone else?

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    6 days ago, # ^ |
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    Can you please tell how to apply network flow here?

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      6 days ago, # ^ |
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      You can build $$$n$$$ nodes on the left and $$$m$$$ nodes on the right, representing the rows and columns. And link an edge from every row to every column, with capacity $$$1$$$. Then link an edge from the source to every row, with capacity $$$a$$$. Also an edge from every column to the sink, with capacity $$$b$$$.

      If the edge from the $$$i$$$-th row to the $$$j$$$-th column is full, put $$$1$$$ on $$$(i,j)$$$.

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        5 days ago, # ^ |
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        Thanks for reply. But i have a doubt. Can you please tell — since the graph will not be unit-graph so complexity will be $$$O(V^2E)$$$ instead of $$$O(EV^{1/2})$$$ for unit-graph. Also edges will be of order $$$V^2$$$ So for $$$1000$$$ test-case time will be $$$O(1000*(50)^4)$$$ but still it's not giving TLE. why? Am i wrong in computing complexity? Please Help.

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          5 days ago, # ^ |
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          Oh it seems I got the complexity wrong. I didn't realise $$$E=n^2$$$ and thought it is $$$50^3$$$ per testcase. I don't know why it isn't giving TLE, maybe on such special graph the complexity is actually lower?

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6 days ago, # |
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Can some cool guy explain the fact that when anyone ask "Is it rated",he must get many downvotes

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    5 days ago, # ^ |
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    Generally, it is written in the announcement if the contest is rated or not.

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6 days ago, # |
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Why do ppl still do that? :) Was that supposed to hide from hack?

=> 81289430

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    6 days ago, # ^ |
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    maybe to not get caught by a plagiarism checker

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6 days ago, # |
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Pretests for Problem D were quite weak..... Lot of problem D solutions were hacked.... Felt demotivated after hacked question, as it affects rankings a lot.....

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    6 days ago, # ^ |
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    I'm so sorry I was upvoting your comment but it got downvoted. What did I do I'm so sorry!

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6 days ago, # |
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What is Test Case #57 for test set 2 of problem F ??

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6 days ago, # |
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Has the system testing finished? If so when will the ratings change?

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6 days ago, # |
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Why am i still unrated even after participating in the contest and given solution 3 problems?

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    6 days ago, # ^ |
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    Cuz they haven't given out the rating updates yet. Look system testing started. It'll happen soon enough I guess.

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      6 days ago, # ^ |
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      Ok I am new to this site . Thanks for helping out

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6 days ago, # |
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If you had felt that the problems are bit difficult for DIV4 you should have put some easy problems in place of those hard problems and make the contest as DIV4. I think the problems are easy for DIV3.

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6 days ago, # |
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Will i be awarded points for successful hacks in this contest... If Yes then how will it be adjusted to the final rankings

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    6 days ago, # ^ |
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    No, you won't be awarded any extra points for a solution hack. It's just that it will affect others' solutions after the system test, and your rank might move up the table (because of others moving down).

    Hacking doesn't yield extra points in Educational and Div 3 rounds (both follow extended ICPC rules for scoring).

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6 days ago, # |
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A very fast system test today after quite some time. Even after the number of participants.

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6 days ago, # |
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My first screencast. Would take suggestions to improve.

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6 days ago, # |
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when will the rating be updated?

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6 days ago, # |
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I would like to appreciate MikeMirzayanov and his whole team for an amazing round. There was no queue during the contest and it ran very smoothly. So it was great work by codeforces team. I hope future rounds are going to be the same one as this.

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6 days ago, # |
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Hey..i am a school student and i just got introduced to codeforces. i took the above mentioned challenge and managed to solve 2 problems, post that my laptop got shut unfortunately. However, both the submitted questions were accepted in the first test as well as post the open hack and system test session. But i haven't got any updates regarding my performance or ratings. I am new to this. Please tell me what am i supposed to do?

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6 days ago, # |
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Dang its been almost 3 hours since the system testing finished, how come no rating update yet?

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6 days ago, # |
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why is my rating not changing even after the final standings of the contest are out

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6 days ago, # |
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for F question. I haven't copied the code and it is entirely different anyways...please resolve the issue asap. I think this may be caused due to the input reader method. Not the logic of the question. manish_98 solution: https://codeforces.com/contest/1360/submission/81281906 my solution: https://codeforces.com/contest/1360/submission/81299107

update : issue resolved by Mike :)

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6 days ago, # |
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rain-maker and return_47 are both my accounts with different gmail id's.I made them to treat one as main and one as practise account hope you understand therefore it's not the case of cheating

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6 days ago, # |
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It's not cheating both are same

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6 days ago, # |
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Similar message appeared to me as well of copying and I have used unfair means to copy from aman_shah username. Sincere apologies and you can take action against me.

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6 days ago, # |
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Funny that my first time I solved 5 problems, I decreased in rate :P

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6 days ago, # |
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Can someone explain how this user went to CM without solving even a single problem?

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    6 days ago, # ^ |
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    I think it's because of changes made for the new rating distribution, and I think they messed up somewhere.

    According to the post, that user should get -100, but since the first contest displays a rating of 500 + (rating that the user should differ by in the contest), the user was able to get +400 rating.

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6 days ago, # |
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Wow, is this even real?

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    6 days ago, # ^ |
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    bug?

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      6 days ago, # ^ |
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      Probably. The latest contest's rating changes are rolled back right now. I hope mine will change to make me an expert :P

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6 days ago, # |
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Why my consts list did not add the const but my rating still changed?

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6 days ago, # |
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What the hell! New Ids becoming Master/International Masters with +800 Rating.

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6 days ago, # |
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I see many success stories on the way titled "How I became an International master in 2 hours "

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6 days ago, # |
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I think some mess has taken place while rating changes... In all the contests the CF rating predictor- CF-Predictor-Herokuapp shows the correct rating changes and every time I have seen the rating predictions here are accurate but in the last div.4 and this div.3, I have seen that the actual ratings are near about -10 than shown in the predictor. In div.2 contests, it works correctly. I think the predictor has some bug while calculating rating changes. In fact in the last div.4 and this div.3, I see that the actual ratings of mine differ by -10 as compared to those shown in the predictor. See the following rating predictions as shown by CF Predictor(the delta column) and the actual rating changes can be found in the above comments.:

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6 days ago, # |
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I have observed that problems in CF Round #644 (Div 3) were quite easy. Except the last, all problems had 2000+ successful submissions during the contest. Due to this, the rating of participant in 1400-1600 exploded. Adding more to this, new accounts became masters just by solving 2 problems last night. LOLzz.

This might be due to new rating system. However, this completely destroys the motive of the new rating mechanism, that was to reduce creation of new accounts.

I propose two points after seeing what happened:

  1. Withdraw the new rating system.

  2. Make prev. round unrated for participants who were in 1400-1600 before the contest. As the problems were indeed suitable for Div 4 contests only.

Comment if you agree/disagree with my proposal.

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    6 days ago, # ^ |
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    agree

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    6 days ago, # ^ |
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    completetly agree

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    6 days ago, # ^ |
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    Totally 100% agree I believe people with ratings between 1200-1400 suffered the most, because even if they performed decently according to what should have been a div4, they got large down plunges in ratings.

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    6 days ago, # ^ |
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    First one is more appropriate

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    6 days ago, # ^ |
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    Disagree. When the problem setters already claimed that this set of problems was more suitable for Div 3, then why debate on it? They have much more experience (along with better coding skills) than us. And, don't forget that among those 2000+ successful submissions, there are also submissions made by unofficial contestants.

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6 days ago, # |
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rating changes are rolled back already

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5 days ago, # |
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I become expert today for the first time..That's a great feeling .

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5 days ago, # |
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From my profile page, Rank displayed is 6204.

Whereas from the contest's standing page, it's 8146. Is this a bug or is there some reason behind it?

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    5 days ago, # ^ |
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    The rank which you are seeing on the contest standing page also includes the particants with rating above 1600(i.e the ones not eligible for Div3)

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5 days ago, # |
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Can u explain me Spy Strings

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4 days ago, # |
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After Participating, I felt it was more suitable for Div 4 rounds. Although its not a big deal!!