DmitryGrigorev's blog

By DmitryGrigorev, history, 4 months ago, translation, In English,

Hi, Codeforces!

We are glad to invite you to take part in Codeforces Round #569 (Div. 1) and Codeforces Round #569 (Div. 2), which will be held on Friday, June 21, 2019, at 19:35. The round will be rated for all participants from both divisions.

Problems for the round have been proposed by Ivan ScreaMood Fedorov, Kyrill Choopa_choops Bessonov, Mukhammadjon Mr.Hakimov Hakimov, Fedor osaaateiasavtnl. Ushakov, Fedor Kuyan Kuyanov, and me, Dmitry DmitryGrigorev Grigoryev.

The round have been prepared by us, Dmitry DmitryGrigorev Grigoryev, Fedor osaaateiasavtnl. Ushakov, Dmitry TheWayISteppedOutTheCar Piskalov and Mukhammadjon Mr.Hakimov Hakimov.

We'd like to give thanks to Ildar 300iq Gainullin for excellent coordination of the round, to Sooke Sooke Gnar, Xiuhan sunset Wang, Ziqian TLE Zhong, Junzhao FizzyDavid Yang, Jiaxuan samjia2000 Gao for testing and to Mike MikeMirzayanov Mirzayanov as well for his unbelievable Codeforces and Polygon platforms!

Participants in each division will be offered 5 problems and 2 hours to solve them. During the round, you will be helping pupils of one usual Moscow school. Score distribution will be announced, traditionally, closer to the start of the contest

Good luck!

UPD Score distribution for Div.2 is standart — 500-1000-1500-2000-2500

Score distribution for Div.1 — 500-1000-1500-1750-2250

UPD2

Thank you for your participation in the contest!

List of the winners of the contest:

Div.2

  1. PSMaoTheKingOfAzerbaijan

  2. atacan

  3. Shayan.Kashefi_A

  4. LiPro

  5. BrutBurger

Div.1

  1. Radewoosh

  2. ACRush

  3. ecnerwala

  4. mango_lassi

  5. neal

My frank congratulations to all the winners!

The editorial

 
 
 
 
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4 months ago, # |
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We hope for a good contest.

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The whole IOI Chinese team tested the round + 300iq coordinated it.

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    4 months ago, # ^ |
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    So we should expect for a very interesting and tricky contest. :D

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      4 months ago, # ^ |
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      We should expect strong pretests and no errors in the contest.

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        4 months ago, # ^ |
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        This comment didn't age well ;p

        I loved Div.2 B's pretests. This was my first time hacking and I could hack 6!

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    4 months ago, # ^ |
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    You forgot Sooke.

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The whole IOI Chinese team tested the round!!!**- **

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    4 months ago, # ^ |
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    I think you should write your own comments

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4 months ago, # |
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Round tested by Chinese.. Hope it will give me an unforgettable experience :)

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    4 months ago, # ^ |
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    It's actually a Russian round, it was tested by the Chinese.

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    4 months ago, # ^ |
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    What exactly do you mean by unforgettable? Don't scare me :P

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      4 months ago, # ^ |
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      It's 0:22 in the morning in China, and the contest will be start in 40+mins. It's very unforgettable, isn't it? (though in another way)

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        4 months ago, # ^ |
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        have a nap in case of brain power outage

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        4 months ago, # ^ |
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        I had just slept to 00:30,and when I woke up and saw the rest time to wait...It should be unfogettable.

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        4 months ago, # ^ |
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        Well, it indeed was unforgettable. B got hacked :/

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" During the round you will be helping for pupils of one usual Moscow school." What does this mean?

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    4 months ago, # ^ |
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    i too want to know....

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      4 months ago, # ^ |
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      pupils -> schoolkid / students.

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        4 months ago, # ^ |
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        pupils -> between 1200 and 1400 rating

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        4 months ago, # ^ |
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        i don't think i can help them in any way , i started coding almost 3 months ago and my rating is falling continuously.... i think i need help ....

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          4 months ago, # ^ |
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          It will increase once you forget thinking about it! (except just after the contest :P)

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    4 months ago, # ^ |
      Vote: I like it +49 Vote: I do not like it

    This is the theme of the contest, the questions will be like "Pupils of a school in Moscow want to perform a certain task, but its too difficult for them, so they are asking for your help..."

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      4 months ago, # ^ |
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      Nice. I am too tired of helping vasya and petya

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        4 months ago, # ^ |
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        Nastya, Tanya

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        4 months ago, # ^ |
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        Should I mention Alice and Bob? I still don't if Alice is a female or male XD

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        4 months ago, # ^ |
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        The characters in these problem statements are more unrealistic than characters in porn movies.

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    4 months ago, # ^ |
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    Main characters of problems will be school pupils/students.

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Xuihan sunset Wang

I think Xuihan is a typo. You may mean Xiuhan.

(Sorry for my poor English)

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4 months ago, # |
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IOI Chinese testers team lol

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A 5 problem contest after long days

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4 months ago, # |
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Nearly 12 p.m at my place. But all love for programming, I will try my best.

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    4 months ago, # ^ |
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    for me it is nearly 12 am, but Im getting into skipping lunch, so not that bad as it is for you

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      4 months ago, # ^ |
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      VietNam and Brazil: 12 hours away ^^

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        4 months ago, # ^ |
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        Now it is almost 2 a.m. for you '-' Are you going to participate?

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          4 months ago, # ^ |
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          At my place, the contest starts at 11.35 p.m and ends at 1.35 a.m. I think it'll be fine for me to participate

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    4 months ago, # ^ |
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    I think you meant 12 a.m. because by convention, 12 p.m. is noon, not midnight.

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4 months ago, # |
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helping kids, noble task.....who's gonna help me help them......lol

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time to think

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4 months ago, # |
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Best of Luck for your first contest as a problemsetter.

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4 months ago, # |
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Whole people wrote the same comment "ioi Chinese testers team" What is happening!

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4 months ago, # |
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I wish positive rating for every participant :)

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    4 months ago, # ^ |
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    This is not possible :)

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      4 months ago, # ^ |
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      if you consider 0 as a positive number it is possible, LOL, just kidding.

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        4 months ago, # ^ |
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        The total rating decreases after every contest, otherwise, ratings will become higher and higher because new users will join Codeforces with the initial 1500 rating points.

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          4 months ago, # ^ |
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          if no new users participate, then the total rating won't decrease

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            4 months ago, # ^ |
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            Also if no users participate, then the total rating won't decrease

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              4 months ago, # ^ |
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              Also if one user participates, then there won't be any change

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            4 months ago, # ^ |
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            i hope no new user to join, else i am already doomed..

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    4 months ago, # ^ |
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    you cleverly wished for yourself. Nice

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    definitely brother :-) .....

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In fact, Sooke's name is not Sooke Gnar. However, he loves Gnar —— The Missing Link very much.

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This time is not friendly to Chinese players

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4 months ago, # |
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Time changed from 19:35 UTC to 22:05 UTC.
Time increased = 150 minutes.
May we also get +150 rating today.

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New Codeforces round, new chance to fall!

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    4 months ago, # ^ |
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    Div2 contests: How many times do we have to teach you this lesson old man.

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So lucky that the contest starts right before I get home

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A delayed contest has never been good for my rating.

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UPS is needed.

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What is the score distribution?

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When reading the comment section ends!!!

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System Test :(

a

->How 30 minutes before contest felt!!

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4 months ago, # |
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Now we have Caseforces!

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The pretest of problem B is BRILLIANT, because I have just made the first TWO Successful Hacking Attempt in my life !

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    4 months ago, # ^ |
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    What was the Hack?

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      4 months ago, # ^ |
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      The first one is 2\n 0 -1 (which get the wrong answer —— 0 -1), and the second one is 3\n -1 -1 -1 (which get the wrong answer —— -1 -1 -1). Hence , I guess the pretest of it is too big for some wrong program (?)

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        4 months ago, # ^ |
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        my program failed at 9th pretest ,can you help me....

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          4 months ago, # ^ |
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          What was ur logic?
          The logic is simple make all non negative numbers negative by operating on them. If n is odd pick the number with highest magnitude and make it posetive.
          Print all the numbers.

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            4 months ago, # ^ |
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            Logic- firstly i converted the array to another array in which the magnitude of each element is maximum (comparing after operation to it's initial valu) .and then found product of all the numbers in the new array. In second step i checked if product is negative then i found the element in the new array which satisfy: 1. It is negative. 2. It's correspondence to given array is positive and not equal to zero. 3. It has least magnitude satisfying above condition. Then i changed that element back to its initial form in the new array. Now product become positive and maximum.

            After that i printed the new array but it failed at 9th pretest ....

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            4 months ago, # ^ |
            Rev. 2   Vote: I like it -15 Vote: I do not like it

            I solved using C Language ,and used long long int for input but it's it shows large product not possible at test case #9 hence failed What to do?

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              4 months ago, # ^ |
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              In this task max product can be (10^6)^(10^5), but max product that u can use in long long int is 10^18. Obviously, it shouldn't work because overflow.

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      4 months ago, # ^ |
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      I got one on 3/n 1 1 -1

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    4 months ago, # ^ |
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    pretests were too dumb, there is no pretest with a[i]=0

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How to solve D?

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    4 months ago, # ^ |
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    A 6x6 can be traversed like this, other cases are also analogous. You can easily show that this method does not use the same vector twice.

    1	23	25	36	14	12	
    3	21	27	34	16	10	
    5	19	29	32	18	8	
    7	17	31	30	20	6	
    9	15	33	28	22	4	
    11	13	35	26	24	2
    

    But most probably this is not the only solution. There can be other ways as well.

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      4 months ago, # ^ |
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      Is it the only simulation or any proper mathematics.

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        4 months ago, # ^ |
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        You could call it intuition.

        First check whether you can always do it for some $$$1$$$x$$$N$$$ grid.

        Hint
        Complete Solution
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        4 months ago, # ^ |
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        Using that method, it's easy to show that once you take a certain vector, you never use it again. That's because the squares on which you could use that vector have already been visited before.

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      4 months ago, # ^ |
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      what if columns are odd sized?

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        4 months ago, # ^ |
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        It does not differ much

        1	23	25	38	36	14	12	
        3	21	27	40	34	16	10	
        5	19	29	42	32	18	8	
        7	17	31	41	30	20	6	
        9	15	33	39	28	22	4	
        11	13	35	37	26	24	2	
        
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      4 months ago, # ^ |
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      nice!

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    4 months ago, # ^ |
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    Starting from (1, 1), move to (n, m), then (1, 2), then (n, m-1), and so on till (n, 1). Then go to (2, 1), then (n-1, m), then (2, 2), then (n-1, m-1), and so on till (n-1, 1). Keep repeating this process. If you reached a row i where row i == n-i+1. Then just from (i, 1) move to (i, m), then (i, 2), and so on till you visit the whole row.

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    4 months ago, # ^ |
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    For solving D, i converted it into 1-D array and then using two pointer i solved it . For eg . 2 3 ==> 2*3 = 6 we are firstly in 1,then i will visit 6 then 2, then 5, then 3 and so on .

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Is div1E an exercise on the proportional cake-cutting + optimizations to pass $$$O(n^2\cdot\log{n})$$$ queries?

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    4 months ago, # ^ |
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    The 3 accepted submissions seems like $$$O(n\cdot log n\cdot log 10^{18})$$$

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    4 months ago, # ^ |
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    Our solution seems to work in $$$O(n \cdot log(10^{18}))$$$ queries

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What is in pretest 9 of problem B Nick and Array?

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    4 months ago, # ^ |
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    I got stuck on the same pretest :(

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    4 months ago, # ^ |
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    Try 2 3 4. Answer should be -3 -4 4

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      4 months ago, # ^ |
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      how did you solve B?

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        4 months ago, # ^ |
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        The only way to increase product is by increasing |a| for each a in array. Only positive numbers can be increased in magnitude by the given method i.e. 4 becomes -5 so |-5| > |4|.

        So first change all positive numbers like this including 0 to -1. Now all are negative. If even sized array, we cant do better. If odd sized change the largest negative number like -100 to 99. This makes product positive

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          4 months ago, # ^ |
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          what is the logic behind taking the largest negative number in magnitude instead of smallest negative number in magnitude

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            4 months ago, # ^ |
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            When you change an element with value -x from negative to positive it becomes x-1. This multiplies the magnitude of the product by (x-1)/x. You want this to be as large as possible, and it gets larger as x gets larger.

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      4 months ago, # ^ |
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      No I don't think so . My answer is -3 -4 4 according to 2 3 4. I think it is 9 -4 -5. the answer should be 9 -4 -5 .

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      4 months ago, # ^ |
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      Oh got it, I keep the least one as it is.

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      4 months ago, # ^ |
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      Hey, how to arrive at such test cases during contests ? Your test case is brilliant.

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    4 months ago, # ^ |
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    take the biggest negative number instead of least.

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      4 months ago, # ^ |
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      I was making this mistake, can you prove your greedy approach ?

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        4 months ago, # ^ |
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        if you are multiplying 2 numbers x and y, such that |x| > |y| then you can visualize it in 2 ways, first adding y x times and second adding x y times. Now if you decrease x then you are just removing 1 y and if you decrease y then you are just removing 1 x. Now you can see which one is optimal.

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          4 months ago, # ^ |
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          Thanks, a. (x-1)*y = x*y — y and b. x*(y-1) = x*y — x And, yes equation a has higher value.

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    4 months ago, # ^ |
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    try this

    3

    -2 1 0

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please help me to identify my mistake in problem B https://codeforces.com/contest/1180/submission/55894819

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    4 months ago, # ^ |
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    6 -1 -2 -3 0 0 0 try getting this case correct, almost all hacks are the similar to this (afaik).

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    4 months ago, # ^ |
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    The order of array is destroyed after sort. Also, the logic is very convoluted. Observe that -a -1 only increases |a| for positive numbers. Try using this fact

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    4 months ago, # ^ |
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    try this test case 3 -4 -9 -8 Your output is -4 7 8

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    4 months ago, # ^ |
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    have you change the element which is equal to zero 4 0 0 0 0 ans should be -1 -1 -1 -1 but your code print 0 0 0 0

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shit that m1 range i took it int got wrong

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I hope to finally become a candidate master :).

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I'm just gonna skip B in future rounds, C is almost always easier for some reason.

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    4 months ago, # ^ |
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    The logic I used for b was much simpler than c imo, (convert all to negative and then convert the least to positive if n%2==1).

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      4 months ago, # ^ |
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      can u explain your logic with this test case ? 5 1 2 3 4 -1

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        4 months ago, # ^ |
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        it has not to be the least negative, it has to be the least negative different to -1

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        4 months ago, # ^ |
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        first you convert all to negative, ie -6 -2 -3 -4 -5 -1. now because n%2 is even, this is the answer. if it was odd, (if converted array is -6 -2 -3 -4 -5), then pick the smallest value and make it positive (5,-2,-3,-4,-5)

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          4 months ago, # ^ |
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          I mean this. 5 is the number of the element. 5

          1 2 3 4 -1

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            4 months ago, # ^ |
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            -2 -3 -4 4 -1. you can look at my code in standings, its the same logic.

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            4 months ago, # ^ |
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            well after making all the integers negative I brute forced but even then I am getting the wrong answer for test case 9!!

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              4 months ago, # ^ |
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              but if u r done brute force then surely u r multiplying all the numbers initially but from that overflow occurs, due to than u cannot get yur answer correct in large value,

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                4 months ago, # ^ |
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                Oh yes,I realized that later.By the way thank you for reply.

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      4 months ago, # ^ |
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      me too

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      4 months ago, # ^ |
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      even that, C you did not have to think anything, just implement it

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      4 months ago, # ^ |
      Rev. 4   Vote: I like it 0 Vote: I do not like it

      Take this case of length 3

      -2 -3 -4
      I think your logic will not work for this case

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      4 months ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      its a very nice solution, a solution I never thought of, not even a slightest bit.

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      4 months ago, # ^ |
      Rev. 2   Vote: I like it 0 Vote: I do not like it

      Edit:nvm i got it

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        4 months ago, # ^ |
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        Hey, you can either look it through an example: (-2,-3,-5) changed to (1,-3,-5) or (-2,-3,4). clearly, the 2nd product is greater.

        Proof:

        Let a1,a2,..., an be +ve numbers in sorted order.

        It would be obvious that:

        (a1-1)*(a2*a3*a4*...*an) < (an-1)*(a1*a2*...*a(n-1))

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4 months ago, # |
Rev. 2   Vote: I like it +130 Vote: I do not like it

Very nice and enjoyable problems!

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4 months ago, # |
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What concept was involved in Div1C ?

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    4 months ago, # ^ |
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    Hint: imagine it as brackets

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    4 months ago, # ^ |
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    Consider suffixes of cummulative frequency of both dishes and people.

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    4 months ago, # ^ |
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    Can you please expand a bit on your solution, because I couldn't think of something with these hints. Thanks!

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      4 months ago, # ^ |
      Rev. 3   Vote: I like it +71 Vote: I do not like it

      Let $$$fstudents(i)$$$ be the frequency of students with money equal to $$$i$$$.
      Let $$$fdishes(i)$$$ be the frequency of dishes that cost $$$i$$$ money.

      We are looking for the greatest j such that $$$\sum_{k = j}^{1000000} fdishes(k) > \sum_{k = j}^{1000000} fstudents(k)$$$.

      We can consider $$$dif(i) = fdishes(i)-fstudents(i)$$$. We want to find the rightmost index $$$i$$$ such $$$\sum_{k=i}^{1000000}dif(k) > 0$$$.

      We can have a segment tree that stores the value of maximum suffix sum of $$$dif$$$ in range. The updates are just adding and substracting 1 from leaves of the tree. We can answer queries in $$$ O(Qlog^210^6) $$$ by binary searching to find the first $$$i$$$ such that the maximum suffix sum is greater than 0. We can do this faster, in $$$O(Qlog10^6)$$$ by the method of descending the tree. I hope i explained it clearly.

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        4 months ago, # ^ |
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        Thanks Charis02

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        4 months ago, # ^ |
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        Why to bother trees ,if we can solve it using brute force. Can't we make it simple? Edit — I tried but got time limit exceeded message .

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        4 months ago, # ^ |
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        What is method of descending of tree.

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        4 months ago, # ^ |
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        Hey, I think I’m missing something simple. Why is it sum(dif(k)) > 0 instead of sum(dif(k)) < 0? If the sum is positive for some i, doesn’t that mean that we have more students than dishes for that i, not that there’s a dish we can take.

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          4 months ago, # ^ |
            Vote: I like it +3 Vote: I do not like it

          Yes you are right, sorry. I meant to write $$$dif(i)=fdishes(i)-fstudents(i)$$$ . Thanks,I'll correct it.

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        4 months ago, # ^ |
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        Hey, one doubt. If you just update the leaves in the seg tree will it be suffice ? Dont we need to update all suffixes of greater length than this ? I.e lazy prop. Bcoz if you just update one suffix then the suffix greater than this length won't have updated value and will have one student or dish less ?

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          4 months ago, # ^ |
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          No, because by updating the leave you update all ranges containing it (standard segment tree update). You don't need lazy prop because you update only one leaf not a range of them.

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          4 months ago, # ^ |
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          code_kit

          By every node $$$nd$$$ storing the maximum suffix sum of $$$dif$$$ in range, Charis02 means suffix with respect to $$$nd$$$, not the whole array.

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4 months ago, # |
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Can we actually solve D like this or is it just luck?

55894627

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    4 months ago, # ^ |
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    It is almost the same solution everyone wrote.

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    4 months ago, # ^ |
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    It's correct

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    4 months ago, # ^ |
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    A clever implementation. I did the same but far more cumbersome.

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      4 months ago, # ^ |
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      Same here. Now it looks like I wasted a lot of time, after seeing this short submission.

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4 months ago, # |
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Delay in System tests?

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4 months ago, # |
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4 months ago, # |
  Vote: I like it +70 Vote: I do not like it

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4 months ago, # |
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Is there a hacking phase? I suppose only Educational rounds have hacking phase.

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    4 months ago, # ^ |
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    Yes, only Educational rounds and div 3 rounds

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4 months ago, # |
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It is really a good contest.

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4 months ago, # |
  Vote: I like it +25 Vote: I do not like it

It happened to me for the first time. My solution was hacked midway between the contest but after a while I got a notification that my solution was correct.

Link : https://codeforces.com/contest/1180/submission/55886651

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    4 months ago, # ^ |
    Rev. 3   Vote: I like it +156 Vote: I do not like it

    I hacked you during the contest with the test case:

    3
    0 -1 -1
    

    After pressing the hack button, I found that it should be an unsuccessful hacking attempt but it turned out to be a successful one

    I thought it's quite strange and I've reported this to problem setters during the contest Few minutes later they replied "under investigation", and your code is Accepted, my hack became an "ignored"

    Actually I'm not sure what's going on lol

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      4 months ago, # ^ |
        Vote: I like it +55 Vote: I do not like it

      Thanks a lot buddy, You sacrificed 50(150) points just for honesty. Cheers.

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      4 months ago, # ^ |
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      But the setters might be having similar system tests as well, so setters please be careful of your system tests for Div2 B

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      4 months ago, # ^ |
      Rev. 3   Vote: I like it +26 Vote: I do not like it

      We had a little bug in our checker and it didn`t work correctly when the answer is $$$0$$$ and the jury answer is different from the participant answer as a multiset. We are sorry for the mistake.

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      4 months ago, # ^ |
        Vote: I like it +76 Vote: I do not like it

      Hello there.

      Sorry, that was definitely my fault, because as a coordinator I missed this case with $$$0$$$ in the checker. There were no similar tests in other hacks or in systests, so everything should be fine.

      My apologies to shpvb, who suffered from my mistake.

      I will be more attentive to these kind of things next time, sorry again.

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        4 months ago, # ^ |
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        No problem buddy, that is totally cool with me.

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        4 months ago, # ^ |
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        Hey 300iq, My submission has not been evaluated yet. Is there any problem?

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          4 months ago, # ^ |
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          Sorry, It got evaluated in the end.

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      4 months ago, # ^ |
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      Also, thank you for your honesty! Without you, we wouldn't notice this bug.

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        4 months ago, # ^ |
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        Thanks for nice contest and interesting problems, too!

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

I think this works for E, but I wrote too many bugs so couldn't get it to work in time :(

Edit: 55909022. Turns out some constant optimization was required too, so I wasn't that close

solution to E
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4 months ago, # |
  Vote: I like it -47 Vote: I do not like it

I think can't pass D because of lines:

if (n > m) {
swap(n, m);
}

I did not find the anti test. Please count my solution as correct if this is the case :(

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    4 months ago, # ^ |
      Vote: I like it -70 Vote: I do not like it

    DmitryGrigorev please look this. I know it looks stupid but I don't want to lose points because of this.

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    4 months ago, # ^ |
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    Give me more minuses. My logic was correct. If it was n <= m code will passed. You are ratists even did'nt read what I wrote. I wish your solutions failed due to checker get Compilation Error on test 99 and etc.

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      4 months ago, # ^ |
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      Lol. If you swap lines, you swap coordinates. So you should also swap coordinates in your algorithm

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4 months ago, # |
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Why the system test still pending?

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4 months ago, # |
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everyone should be given positive bonus rating because of unexpected power cut occurred before start of exam ..... it mentally affected many people ,hence not giving their best output in exam.....

note: if some one don't like it ,kindly don't down vote it as i already have negative contribution, just ignore this comment and move on.

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    4 months ago, # ^ |
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    Actually it made me feel lucky, due to wrong timing I thought contest will start 1 hour later than actual time

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    how do i delete my comment????

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    4 months ago, # ^ |
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    The power companies should give everyone free money because of unexpected power outage... it mentally affected many people.

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4 months ago, # |
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Any hints for Div1 D?

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4 months ago, # |
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How to solve DIV2 E ?

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4 months ago, # |
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When your rate reach the buttom look at the bright side there's one way to go (me trying to motivate myself after my stupidity streak this month)

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4 months ago, # |
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Div2B have bad pretests.

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    4 months ago, # ^ |
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    And a lot of people had wrong solutions, FeelsBadMan

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    4 months ago, # ^ |
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    Strangely enough, almost always in Div. 2 problem B has the worst [pretests passed / accepted] ratio. Not sure about the reason for this phenomenon.

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    4 months ago, # ^ |
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    It's a comparative thing. Before system test, my standing was 618, after system test, it's 557. So if you avoid all pitfall but others not, you can have a better standing. The contestants who found the pitfall and avoided it should have an advantage over those don't.

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4 months ago, # |
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Good old five-problem contest. Skill over speed. Thank you.

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    4 months ago, # ^ |
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    I like this grand old way of 5 problems. I personally don't enjoy subtasks and contests which has very large problems like 7 — 8+. less and good questions are good to stay you focus during the contest, rather than moving here and there to solve subtasks.

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    4 months ago, # ^ |
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    I doubt it's possible to make skill over speed contest if it lasts less than, at least, 2h30m. When contest lasts a bit longer even if it has more problems in it, there's much more bigger probability that you will reach problems you can't solve because of your skill and not speed before the contest time ends.

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      4 months ago, # ^ |
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      By the end of the 2 hour contest, I often don't reach the task that I'm not able to do, because it takes me a lot of time to solve the tasks before that one, but I really don't think my slow performance should be rewarded with longer contests.

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4 months ago, # |
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Hackforces

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4 months ago, # |
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Please make pretests not so strong next time :)

It seems like there are no fails on systests at all

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    4 months ago, # ^ |
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    Are you really suggesting they should intentionally make pretests weaker?

    Imagine failing to system tests because the specific corner case your code didn't handle was removed from the pretests, while others fail pretests and can fix their code. Yeah, no thanks, more rounds like this please.

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      4 months ago, # ^ |
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      I suggest that pretests shouldn't cover, like, everything.

      So it's more about they shouldn't make them intentionally strong.

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    4 months ago, # ^ |
    Rev. 3   Vote: I like it +18 Vote: I do not like it

    If you are bored with nothing to do, I wouldn't mind knowing what's wrong with my D. :)

    EDIT: I just mean I want someone to debug my code :)

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      4 months ago, # ^ |
        Vote: I like it +91 Vote: I do not like it

      Well, it clearly says that.

      wrong answer 1st numbers differ — expected: '204997350968', found: '204998250971'

      You output incorrect number.

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        4 months ago, # ^ |
          Vote: I like it +8 Vote: I do not like it

        Brilliant explanation on how the WA verdict works! You should totally write a blog on that.

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      4 months ago, # ^ |
      Rev. 4   Vote: I like it +13 Vote: I do not like it

      Your code only computes the correct answer for paths through the first centroid found. This seems to suffice for almost all cases (as Radewoosh mentioned), but not always when the tree has two centroids (which test case 103 satisfies). Try comparing my getAns function with your get_subtree function.

      EDIT: As socketnaut mentioned, there are also counterexamples where the tree has only one centroid.

      (mine) 55894131

      (yours) 55893926

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        4 months ago, # ^ |
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        Can you please tell if my solution for D is correct?
        The final structure will look like a cycle with branches coming out of the nodes which are a part of the cycle. Let there be 'x' nodes in the cycle and size of the subtree going out of these x nodes be a_1,a_2,...,a_x. So now there will be an extra path from any node in one subtree to other subtree. Thus there will be a_1*(a_2+a_3+...+a_x) + a_2*(a_3+a_4+...a_x) +...+ a_x-1*(a_x) extra paths. This can be written as: ((a_1+a_2+...+a_x)^2 — (a_1^2+a_2^2+...+a_x^2))/2 . First term is equal to n^2. So we need to minimize 2nd term.
        So now choose arbitrary root. Let there be a node 'u' with 2 of its children u_1,v_1. let say we want to connect u_1 , v_1 then 2nd term of above equation will become u_1^2+v_1^2+(n-u_1-v_1)^2 . Similarly if want to connect a child of u_1 say u_2 to v_1 then the 2nd term will become u_2^2+(u_1-u_2)^2+(n-u_1-v_1)^2. This expression is less than that if we connected u_1,v_1. So we can say by induction that for any node 'u' we will want to connect 2 of its leaves such that their lca is 'u'. Now if sizes of subtree of node 'i' is denoted by s_i then we can define 2 values for each node: d_i=min(d_i,d_child+(s_i-s_child)^2) and f_i=d_i+(s_i)^2-2*n*s_i. We can show that the second term of above equation can be written as:
        if a node 'u' has only one child: n^2+f_child.
        if a node 'u' has more than one child: n^2+ f_child1 + f_child2 +2*(s_child1)*(s_child2).
        Now our equation was telling us number of extra paths on adding an edge, so it must be positive and for any node 'u' if we subtract the second term from first we get -(f_child) or -(f_child1 + f_child2 +2*(s_child1)*(s_child2)) respectively which means that f_i will be negative. So for any node if we choose 2 of its children such that their f_i are smallest among all children we can get the max number of extra paths added. So we find this for all nodes and find their max.

        My solution:55909246

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    4 months ago, # ^ |
      Vote: I like it -18 Vote: I do not like it

    Better call GreenGrape

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4 months ago, # |
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I have only looked at problem C so far, but it is already enough for me to conclude that it was a brilliant round. Thanks guys!!!!!!!!!!!!

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Wasn't Div2-C easier than Div2-B ?

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    4 months ago, # ^ |
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    (Give me negative vote).Div2D was easier than B and C.

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System testing completed good rate for everyon.

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4 months ago, # |
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pctr

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Problem D of Division 2 was very interesting and stumped me. I'm excited to read the editorial. Thank you for the great problems!

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I've never waited so long for systest/rating changes!! Good round tho :-)

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11 and counting :P

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4 months ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

Simple solution for Div2 B.

Submission Link : https://codeforces.com/contest/1180/submission/55907361

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4 months ago, # |
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can someone help me in Div2 problem B.

I am getting wrong answer in test_case #60.

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Any hints for div2C? Thanks :)

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4 months ago, # |
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Is this true that in d1d optimal path will contain the centroid of the tree? Does anybody have a proof/a counterexample?

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    4 months ago, # ^ |
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    What do you mean by optimal path? Could you share some hints about how to solve the problem?

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      4 months ago, # ^ |
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      By optimal path I mean the path between vertices which will be connected with an additional edge. The hint is that if we group the vertices by the vertex on the path which is closest to them, only the sizes of the groups matter somehow (you have to minimize the sum of squares of these sizes).

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    4 months ago, # ^ |
    Rev. 2   Vote: I like it 0 Vote: I do not like it

    Hmm, don't know about centroids, but there always is an optimal path which connects two leafs.

    (Guess it's obvious though)

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    4 months ago, # ^ |
    Rev. 2   Vote: I like it +115 Vote: I do not like it

    It's false. This type of graph produces a counter-example:

    There are $$$N = W + 2T + 2$$$ nodes. When $$$2(W + 1) > N$$$, $$$C$$$ is the unique centroid.

    The orange edge produces an optimal path going through $$$C$$$. The number of simple paths when adding it is $$$2 { N \choose 2 } - { T + 1 \choose 2 } - { W \choose 2 }$$$.

    The blue edge produces $$$2 { N \choose 2 } - { W + 2 \choose 2 }$$$ simple paths. Any choice of $$$W$$$ and $$$L$$$ such that $$$C$$$ is the unique centroid and $$${ W + 2 \choose 2 } < { W \choose 2 } + { T + 1 \choose 2 }$$$ produces a counter-example.

    $$$W = 17$$$, $$$T = 8$$$ is one such choice.

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    4 months ago, # ^ |
    Rev. 5   Vote: I like it +81 Vote: I do not like it

    After reading socketnaut's comment, I wondered that what is the minimum case which all of optimal path does not contain any of the centroid of the tree.

    I wrote a brute-force code, and proved that there is no such case which is $$$N=3,4,5,6,7,8,9,10$$$ and $$$11$$$. Seems like socketnaut's counter-example ($$$N=35$$$) could be the minimum.

    Source Code of Brute Force: Link

    UPD: Finally I realized that socketnaut's second comment means my user photo :)))))))

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      :) "Who is this man and why is he trying to sell me this graph."

      I have some intuition for why this form might produce the smallest possible. Adding leaves to $$$C$$$ is the best way to make it be the centroid without incentivizing the best path to go through $$$C$$$. Or to put it a different way, if the best path doesn't pass through $$$C$$$, we can reformat all of the subtrees that are below $$$C$$$ and away from the best path into leaves attached directly to $$$C$$$. Such a change doesn't worsen the best path and doesn't improve the paths going through $$$C$$$.

      Going to the best path, suppose that the blue edge connects some vertices $$$u$$$ and $$$v$$$, and $$$\ell$$$ is their LCA when the tree is rooted at the centroid. If either $$$u$$$ or $$$v$$$ has a "worse" path coming down from $$$\ell$$$, the orange edge can always connect the centroid to the one that is better. So, we need them to be symmetric. Minimizing the path between $$$C$$$ and $$$\ell$$$ and making the trees from $$$\ell$$$ down to $$$u$$$ and $$$v$$$ linear gives the best configuration.

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I was unable to solve Div2 Problem B, which was failing pretest #9 repeatedly.
I am still unable to find my mistake. This is my submission 55907969
Please help me to identify my mistake

Thanks!

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    4 months ago, # ^ |
    Rev. 2   Vote: I like it 0 Vote: I do not like it

    what is the maximum product for this case

    3

    1 0 -1

    I think it should be
    -2 -1 -1 but in some successful submit code i found this one as output

    1 -1 -1

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4 months ago, # |
  Vote: I like it +9 Vote: I do not like it

My solution to Div. 2 D TLEed on TC74 when compiled with MSVC++ while the same exact solution ran in 280ms when compiled with G++. A custom test shows that my solution ran in 1029ms when compiled with MSVC++. Is MSVC++ really that much slower than G++?

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    4 months ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Ah that's sad. Probably the constraints were too tight. Even I got TLE & afterwards got an AC just by replacing all cout's with printf.

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      4 months ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      Agreed. I was only able to get AC using VS by using printf, not even cout with ios_base::sync_with_stdio(false) ran in time, which I previously thought was only slightly slower than printf. I can't think of why the constraints need to be this tight, since as far as I can tell there isn't an easier solution that needs to be prevented from passing.

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

I am not sure if this is a right place to put my idea/complaint . I am from Beijing,Chine, and in the contest last night , I refresh codeforces.com and m2.codeforces.com for 25 minutes only to find a series of "This site can’t be reached". And this situation is common before. According the comments following the blog of Round569 , people out of China didn't encounter this annoying process. So , I write this wanting you to know what happens in China and hoping for a response. Maybe you buddies can help me to let worker in codeforces know the situation? :)

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    4 months ago, # ^ |
      Vote: I like it +11 Vote: I do not like it

    Try codeforc.es

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      that site works perfect , with only few seconds to load , which is uncommon in cf. thanks!!!

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I am from Shanghai China, I did not encounter the situation you said. Maybe it's because the Great Fire Wall. Because of 70th anniversary of China, the censorship is strong now. I use VPN to visit Codeforces. Try using VPN.

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      待我肉身翻墙日,youtube上把党夸

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4 months ago, # |
Rev. 2   Vote: I like it +13 Vote: I do not like it

I should try this round because I think I can solve all problems in div.2 , but I am afraid that I will get lower rating. Maybe I should be braver.

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    4 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    In the short run, your rating may fluctuate. But in the long run, your rating converges to your skill. Participating and practicing more improves skill. So just calm down and focus on the long term performance. That's how I convince myself to not afraid of a drop in rating and participate as many as possible. Hope that helps.

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Hi. So I was wondering about my submission for Problem B here.

The verdict says 'wrong answer multiply isn`t maximal possible', but not something like 'expected X but found Y'.

So, I just want to know whether there is some issue here? I don't see where my submission is wrong

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    4 months ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    I have also done the same mistake , that you dont have to choose only positive max element. but negative max element (in absolute value) can be better choice .

    Eg -5 2 3 Here answer will be 4 -3 -4

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      4 months ago, # ^ |
      Rev. 2   Vote: I like it 0 Vote: I do not like it

      Oh no! I messed up the compare function. Should have payed more attention while coding it.

      UPD: Changing compare function still doesn't work sadly :(

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4 months ago, # |
Rev. 2   Vote: I like it +12 Vote: I do not like it

Again Editorials are late.

This is actually becoming very common these days. Earlier the editorials were published right after the system tests, but now they come really late.

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Just read this thread/look at the code in standings, you don't have to wait for the editorial.

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4 months ago, # |
Rev. 2   Vote: I like it +88 Vote: I do not like it

the editorial flies away~~  pigeon is walking~~~

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4 months ago, # |
  Vote: I like it -6 Vote: I do not like it

Good

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4 months ago, # |
  Vote: I like it +4 Vote: I do not like it

So where is the Editorial

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4 months ago, # |
  Vote: I like it +3 Vote: I do not like it

if my submission got hacked, then how to know for which test case the submission is wrong??

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4 months ago, # |
  Vote: I like it +3 Vote: I do not like it

How to solve Div 2(Problem E)??? Anyone..thankyou

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Problem D: I submitted Submission 1 but it was showing 'TLE'. But then I again submitted by just changing 'endl' to '/n' and it got submitted. Accepted Submission. Please explain why this happened?

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