awoo's blog

By awoo, history, 6 months ago, translation, In English

Neapolis University Pafos

Hello Codeforces!

The series of Educational Rounds continues thanks to the support of the Neapolis University Pafos.

On Nov/14/2025 17:35 (Moscow time) Educational Codeforces Round 184 (Rated for Div. 2) will start.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest, you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

The problems were invented and prepared by Adilbek adedalic Dalabaev, Ivan BledDest Androsov, Maksim Neon Mescheryakov, Roman Roms Glazov, Maksim FelixArg Novotochinov and me. Also, huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

UPD: Editorial is out

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Hope to reach expert again with this contest.

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Hope to finally hack LonggVuz

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hoping +delta

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Could you please add the score distribution :)

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I hope to solve atleast a single question. :)

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Hope to solve at least 3 problems, thus increase my rating.

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Long time no see

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Hope we will have a good night with this round.

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May I ask who can tell me the difficulty of this competition plz orzorz

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"6 or 7 problems"

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WHY UNRATED ALLOWED???

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Is the contest delayed?

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Am I the only one for whom the contest hasn't started ??

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Why is it getting delayed over and over again?

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I don't know if it's a glitch or not , but whenever i am refreshing my website its showing that contest will start after 11 min 6 seconds. I am not able to enter in the contest even though i have registered in this contest.

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What happened to Codeforces? ;((

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weird .. I tried entering the contest but couldn't the timer refreshed again and again ..

and then I entered, and looks like I am late, others have already made submissions :cry

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    6 months ago, hide # ^ |
     
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    But your submissions don't show todays contest

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      6 months ago, hide # ^ |
       
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      yes, I haven't made any submissions yet, as I got late because of this timer glitch ..

      but I can see standings and the number of submissions others have made to a specific problem ...

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is it just me or does the timer keep refreshing on the 11th minute?

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    same for me.... I have left one page where I will not refresh and let the timer go to zero .... we will see what happens

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The contest seems to be working from m1.codeforces.com but keeps getting delayed in the main page. I found that out 15 minutes into the contest. What a waste of time, now I cannot answer the contest as that would lower my rating.

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    6 months ago, hide # ^ |
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    I guess you can submit... with very high probability ( coz many people facing issues ) .. this round might end up unrated .. but if you don't want to take risk that is OK !!

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The same thing happens when I don't log in or access it via mobile. What is this?

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ok now suddenly it says contest is running and I can see the problems ..

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This round should be unrated.

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    It will be unrated, it has to be

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      6 months ago, hide # ^ |
       
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      Why? Those who couldn’t enter couldn’t submit anything.

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        6 months ago, hide # ^ |
         
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        it seems, it started at different time for different people, so possible that many people started late and they didn't know others started earlier.

        like people were getting their start timer reset to 11.06 on refresh as mentioned in other comments and were not entering the contest at the scheduled time ( I faced this too )

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    Obviously it should be

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honestly , the 11:06 is so irritating and I'm so late that I gave up the contest ╯︿╰

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I was looking forward to reaching the specialist rank today, but unfortunately the contest started and I couldn't participate because Codeforces kept saying 'starts in 10 minutes'. gg :(

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contest submission page is also misbehaving for me ( not showing latest submissions )

[workaround] .. so I am able to see updated status on personal submission page on my main profile submission

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I don't like this round

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LOL!!!

now contest remaining time also shows different value on different pages,

after refreshing them now

  • home page of codeforces showing 25min remaining
  • dashboard of contest shows 4min remaining
  • standings page shows 55 min remaining

I think some issue with timer of codeforces today LOL .. I am on main codeforces site and not a mirror

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one of the least enjoyable contests in recent memory

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That sigh of relief after having solved D2 and D1 simultaneously in the last 5 minutes !!

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UGH D2 TLE on test 27

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D2 TLE on testcase 27 :sob:

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round was not good in overall experience,

I hope this doesn't happen on Sunday, as I cancel some of my outdoor activity to make time for this

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What is the reason to reject binary search solution in D2 =(

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Can anyone give some advice for C?

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    I guess we need to solve (l+r)*(r-l+1)>prefix[r]-pre[l-1] but I couldnt figure out the implementation part in the given time, will have to upsolve.

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    For a fixed $$$l$$$, we need to calculate

    $$$ \displaystyle\max_{r} [p_{l-1} + (r+l)(r-l+1) + s_{r+1}] \\ = \displaystyle\max_{r} [p_{l-1} + r^2 - l^2 + r + l + s_{r+1}] \\ = p_{l-1} - l^2 + l + \displaystyle\max_{r}[r^2 + r + s_{r+1}] \\ $$$

    So you can just precalculate $$$[r^2 + r + s_{r+1}]$$$ values and their suffix max, then iterate over all $$$l$$$ and calculate the value for each.

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    The nice solution: consider the sequence $$$b_k = a_k - 2k$$$. Note that the sum $$$\sum_k a_k$$$ which we wish to maximise is equal to $$$n(n+1) + \sum_k b_k$$$ and that setting an interval $$$a_l, a_r$$$ to $$$l+r$$$ is equivalent to setting an interval $$$b_l, b_r$$$ to $$$0$$$. Therefore, the interval that solves the problem for $$$a$$$ is the interval in $$$b$$$ with minimal sum (which we subtract from the total sum by deletion). This is a standard problem.

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      6 months ago, hide # ^ |
       
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      can you please explain equivalence of given operation to setting b[l:r] to 0

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        Notice that the operation on $$$a_l$$$ to $$$a_r$$$ is adding $$$(l-r+1)(l+r) = 2(l+\ldots+r)$$$ (this equality is the key step!) and subtracting $$$a_l+\ldots+a_r$$$. Therefore we can rewrite it as subtracting $$$\sum_l^r (a_k - 2k)$$$ from the total sum of $$$a_k - 2k = b_k$$$.

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          6 months ago, hide # ^ |
           
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          whoa thanks, I can see it now!!

          this idea is so cool.

          thanks for sharing your genius approach

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    dif(l, r): change in sum of the array after applying the operation in l, r.

    let's say we have dif(l, r — 1) calculated for all l and we want to calculate dif(l, r)

    dif(l, r) = dif(l, r — 1) + (r — l) + (l + r — a[r])
    = dif(l, r — 1) + 2*r — a[r]

    (r — l): every value in the previous interval increases by 1
    (l + r — a[r]): current change

    so, if we choose to add some element in the operation, the change in difference doesnt depend on the previous values

    thus, the answer is gonna be the sum of the original array + largest subarray sum of b where b[i] = 2*i — a[i]

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    xyz

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I guess the codeforces servers still loves me..

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Please check my approach to B. https://mirror.codeforces.com/contest/2169/submission/348983371 Some edge case i miss or whole approach is wrong!!

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I think codeforces should add an option to report cheaters in a contest.

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D1 had a feeling similar to josephus problem .. so made some weird recursion kind of solution but couldn't optimize for D2 .. guess some observation (like different approach for smaller and larger y ?? ) and then math stuff ??

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    just binary search worked for me

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      ok like binary search the answer A and then in validity check .. try to check how many numbers smaller than A were crossed out ... something along that line ??

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        6 months ago, hide # ^ |
         
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        keep subtracting A / y for x times. see if the remaining is larger than k.

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          for D2 ??

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            D1. I got TLE on D2. still no clue

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              6 months ago, hide # ^ |
               
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              yeah I did something weird for D1 .. no idea of D2

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              6 months ago, hide # ^ |
               
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              The key idea is that one operation transforms a position p into

              p + ((p-1)/(Y-1)).

              However, the added value d = (p-1)/(Y-1) stays constant while p remains in a specific range. Instead of considering it one at a time , we consider jumping(adding) multiple such d . This jump length is (R − p)/d + 1 (we can prove it) , where R =(d+1)(Y−1).

              We subtract whole blocks from X and increase p , it can be seen that no more than 50-55 such jumps we need

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                6 months ago, hide # ^ |
                 
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                oh wow !! how come you are specialist

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                  6 months ago, hide # ^ |
                   
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                  he isnt

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                  6 months ago, hide # ^ |
                   
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                  what do you mean .. alt account ?

                  btw I am just saying that I didn't get an idea as someone rated expert so I was just kind of appreciating the idea .

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                how do you get to this point , have you seen such kind of jumping before ?

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                  6 months ago, hide # ^ |
                   
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                  we can always think of it repeating a constant task one by one is always bad than doing it in bulk at a time !!

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                Can you elaborate on why we need only 50-55 jumps (i.e. log 1e12)?

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                This is nice . How u come to this conclusion ?

                i mean experience or just trying out in live contest[love to hear].

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    Suppose we have a position $$$p$$$. If $$$p \bmod y \neq 0$$$, then it changes as:

    $$$ p \rightarrow p - \left\lfloor \frac{p}{y} \right\rfloor $$$

    The non trivial idea is as follows: We start from a position $$$k$$$, and we want to find the original position $$$p$$$ that was transformed into $$$k$$$.

    Let $$$k = q \cdot y + r$$$ (where $$$0 \leq r \lt y$$$). If $$$q + r \lt y$$$, then we can simply perform:

    $$$ k \mathrel{+}= q $$$

    This operation directly moves $$$k$$$ to the end of the current interval. If this process stays within bounds (does not spill over the interval), you can repeat it as long as the condition holds.

    Once $$$q + r \geq y$$$, we want to find $$$q_2$$$ such that:

    $$$ k + q_2 = q_2 \cdot y + r_2 $$$

    which rearranges to:

    $$$ k = q_2 \cdot (y - 1) + r_2 $$$

    Here, $$$r_2 \leq y - 1$$$. To check if $$$r_2 = y - 1$$$, simply verify:

    $$$ k \bmod (y - 1) = 0 $$$

    now , solve for $$$q_2$$$ and update $$$k$$$:

    $$$ k \mathrel{+}= q_2 $$$

    By applying these steps, you obtain an algorithm that works in $$$\sqrt{x}$$$ steps.

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      ok, thanks.. I will try to understand more and upsolve later

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      I tried this exact approach once i noticed the quocient was repeating, but still got TLE on 25. do you have any idea why? I couldn't see any edge case.

      I mean it was quite of a reach, overall solution should work on $$$\mathcal{O}(T\log(10^{12})$$$$$$\sqrt x$$$), which is around $$$10\cdot40\cdot10^6 = 4\cdot10^8$$$. Did yours get AC?

      https://mirror.codeforces.com/contest/2169/submission/348965036

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        why are you doing binary search ? If you read what I do ,there is no binary search involved.

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          The idea was the same "We start from a position $$$k$$$, and we want to find the original position $$$p$$$ that was transformed into $$$k$$$.", i tried to solve this using BS, then my check was supposed to find the final position for a given number $$$p$$$. Now I'm not so sure I understood your solution, it is really similar to what I've done in my check function. Would you care to explain a little further how is it that your steps end up finding $$$p$$$?

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            6 months ago, hide # ^ |
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            you simply start with $$$k$$$ , and find what next position $$$p$$$ satisfies $$$p \mod y \neq 0$$$ and transitions to this $$$k$$$ , and then update the $$$k$$$. You can look at my submission.

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    xyz

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How to d2? I solved d1 by binary searching over if there exists an element $$$ c $$$ such that at least $$$ k $$$ elements survive $$$ \lt = c $$$ after all operations.

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My $$$E$$$ solution:

First, $$$ans=sumc-minc$$$.

Considering only the point $$$i$$$ in the lower left corner and the point $$$j$$$ in the upper right corner, the answer is $$$max(sumc + (x[j] + y[j] - c[j]) + (-x[i] - y[i] - c[i]))$$$. Note that absolute values are not required here, as some "erroneous values" will not exceed the actual maximum (the same logic applies as in https://mirror.codeforces.com/contest/1859/problem/E).

Therefore, we only need to preprocess $$$mx1 = max(x[i] + y[i] - c[i])$$$ and $$$mx2 = max(-x[i] - y[i] - c[i])$$$, then update the answer with $$$sumc + mx1 + mx2$$$.

It is also possible that the lower left and upper right corners are not determined by a single point but by two points together. We only need to similarly record $$$mx3=max(x[i]-c[i]), mx4=max(y[i]-c[i]), \ldots$$$ and update the answer.

It is also possible that the rectangle is determined by the top-left and top-right corners. We don't need to do more caseworks; simply multiply all $$$x$$$-coordinates by $$$-1$$$ and run the solve function again.

AC code: 348984311

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    Pls put it under spoiler because it spoils the problem for ppl just wanting to read general comments here

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There is a problem that is similar to the D2 solution. https://mirror.codeforces.com/gym/105588/problem/C

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D2 is too mysterious.

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d1 is similar to 1852A - Ntarsis' Set

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Sol D?

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My code for C passed but i don't have proof

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this contest b ; i had the proper idea in 10 mins after start but forgot the cases in which the counter resets but has * as past state and was frothing at the screen like a code monkey wondering what went wrong and just managed to find that 1 min after contest ended ; hopefully i learn something for the future ;

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348953962

Why does my code get a TL? I am jumping over the steps like in this problem

CSES

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Problem D seems awfully similar in concept to https://mirror.codeforces.com/contest/1852/problem/A , the binary search / two pointer in the question linked is changed here to math due to being multiples of a single integer but doesn't change the solution significantly. Just wanted to make the authors aware about it.

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    It's similar to D1, but not D2.

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      people are hating it (especially D2 just because they couldn't solve it).

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      The problem itself yes but if you open the editorial you can see the linked problem was originally intended with number of ops upto 10^9 and the solution posted is also for that which matches D2.

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D1 seems to be simpler than C

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RP++ :))))

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Here is my live Screencast with detailed explanation for [A-C]. Video link : YT Video (It's in Hindi Language).

I may upload explanations for D2 and E if anybody wants.

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    yes please D2 and E will be helpful

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    Just Watched your solutions for A (visualization approach) & B, aswell, pretty nice, Thanks !

    I just overcomplicated A For some reason, visualizing helps to solve quick for sure !

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One of the first div2 constest from a long time where B was actually easy and wasn't harder than C i think

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I unnecessarily did a trash ahh dp in C

solution

Solution link: https://mirror.codeforces.com/contest/2169/submission/348964665

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The Hack of problem A is the most serious attack in ex-ICPC rule because the score of every problem is equal, but most people gain the smallest penalty in A. Never make weak tests in A please.

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nuh uh, where is the editorial?

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another mathforces

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I participated in Educational Codeforces Round 184 (Rated for Div. 2) and managed to solve 3 problems.

Huge thanks to awoo, adedalic, BledDest, Neon, Roms and FelixArg for preparing this contest — really appreciate the time and effort!

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6 months ago, hide # |
 
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dig.

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6 months ago, hide # |
 
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You can see that problem D2 is highly similar to problem C from ICPC 2024 EC Kunming, and I think this is unfair. https://qoj.ac/contest/1871/problem/9864

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6 months ago, hide # |
 
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Okay fine... I might be stupid for missing the "=" edge case in A in my solution and I deserve to be hacked, but I'll excuse myself as I only had 3 minutes total to get the problem accepted so not much time thinking... But how'd the problem creator miss it?! :S

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6 months ago, hide # |
 
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Is there a system testing for this contest?

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6 months ago, hide # |
 
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Why F using 5*log times NTT lead to TLE?Is my NTT too slow or just having another method?

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6 months ago, hide # |
 
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Isnt the hacking part over? When will the ratings come? Or because of the some people had problems in the beginning the contest turned into unrated?

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6 months ago, hide # |
 
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Has this contest become unrated , i dont know why i didnt get rating update****

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6 months ago, hide # |
 
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Add DP to problem C, as can be shown here:

https://mirror.codeforces.com/contest/2169/submission/348989596

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6 months ago, hide # |
Rev. 2  
Vote: I like it -8 Vote: I do not like it

I love this game!

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6 months ago, hide # |
Rev. 2  
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1

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6 months ago, hide # |
 
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why unrated?

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6 months ago, hide # |
 
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Just update the ratings man, i did 3 questions and i was so happy that I’d finally become pupil and now this because of a few people?? If they had a issue they could’ve simply skipped the contest, why should the round be unrated because of a issue like this?and no official response either

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    6 months ago, hide # ^ |
     
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    Your first code is hackable, I hope you are aware of that. You will become a pupil in this round, so do not worry.btw congratulations in advance.

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      6 months ago, hide # ^ |
       
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      How is my code hackable? Also apparently we’re from same college lmao

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        6 months ago, hide # ^ |
        Rev. 2  
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        You should have totally ignored elements equal to A (don't count them at all); and then it would have been a correct solution.

        Spoiler
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6 months ago, hide # |
Rev. 5  
Vote: I like it +11 Vote: I do not like it

It's an educational round with a 12-hour hacking phase. So it's normal for it to take 16 to 24 hours to announce the final rating. Keep patience; it will be announced soon.

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6 months ago, hide # |
 
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When will the change in rating be reflected?

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6 months ago, hide # |
 
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TUTORIAL PLZ

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6 months ago, hide # |
 
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I think user testerAbstract cheated

He use if (!(cin >> t)) is one of chatGPT's coding style

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    6 months ago, hide # ^ |
     
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    i agree with you not only that he used different typenames throughout the contest, sometimes he will use long long, sometile using ll = long long, sometimes i64=long long, and then using unit_32t instead of long long for quetion F which is like okay for ntt but not particularly needed but chatgpt still uses it, moreover he sometime uses t and T for testcases sometimes with if(!(cin >> t)) sometimes without, and i refuse to believe he wrote entire templates for each code and got this, there is literally no consistency in his perf and coding style alike. he is a cheater

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    6 months ago, hide # ^ |
     
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    Agree. He has even got rank 1 and reach 2100+... There's no meaning to refer cf rating anymore.

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6 months ago, hide # |
 
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are there some editorials for this ?

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6 months ago, hide # |
 
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how to know what input hacker used for our code?

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6 months ago, hide # |
 
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Is it normal for system testing to be stuck at a specific percentage for longer than average? Just curious :3

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6 months ago, hide # |
 
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1h ago: System testing (95%)

now: System testing (95%)

My D2 submission: In queue

Cannot see my contest submissions in Contest -> My Submission page, only non-contest ones are visible

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    6 months ago, hide # ^ |
     
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    You mean people who did not participate in the contest and are now making submissions are having higher priority in queue than the ones who did participate in the contest? :(

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      6 months ago, hide # ^ |
       
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      this is standard, my guess is that many bots are spamming submissions which is causing a delay in sys tests

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        6 months ago, hide # ^ |
         
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        I feel like the system is flawed here, first they should prioritize contestant submissions and get the results published and then deal with submissions that are made by folks who did not participate

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          6 months ago, hide # ^ |
           
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          you feel like that rn, but there is nothing you can do to change your contests submissions, people practicing need priority because they are currently working on the problems, system testing does eventually finish so no change whether it happens right away or 2 days later. usually they dont take this long really unfortunate, but the system is correct.

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      6 months ago, hide # ^ |
       
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      I don't know the details, but the progress of system testing seems get stucked for ~1h, and my D2 hasn't been judged till now. Hopefully things will get right soon.

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        6 months ago, hide # ^ |
         
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        Yeah i'm hoping to reach pupil with this contest, though I'm not quite sure whether my solution to C will pass the tests or not.

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        6 months ago, hide # ^ |
         
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        i see, it becomes slower than before, after it reachs 90%

        By the way, I realllllllllllllllly like your profile picture, i love clannad, i like nagisa!!!!

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6 months ago, hide # |
 
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System testing rn : pic

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6 months ago, hide # |
 
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Why's it stuck on 95%?

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6 months ago, hide # |
 
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stuck is like my mind and code

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6 months ago, hide # |
 
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when will the ratings be published?....for 6 hours its showing 95% system testing

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6 months ago, hide # |
 
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Why's is system testing stuck on 95% from last few hours :(

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This has to be one of the worst contests bro...

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system test at 5 hrs before — 95 % system test now — 95% system test 1 light year later — 95%

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We are getting TL on system test :(

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Was this match unrated?

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6 months ago, hide # |
 
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If someone registers for a contest but doesn’t actually participate, does their rating stay the same? My friend forgot to join this contest, and his rating didn’t change. Or maybe that only applies to Educational contests?

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Rev. 2  
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Hello Codeforces Team,

My handle is rakshithnayakam08, and I recently participated in Contest 2169 (Educational Codeforces Round 184). My participation was marked as skipped, but I would like to request a review because I believe I met the contest rules and did not use any unfair means. Here’s what happened and why I believe the “skipped” status is incorrect: I actively solved problems during the contest, specifically I attempted problems A,B,C. I coded the solutions myself without any external help or collaboration. My logic and code style are consistent with my previous submissions. If needed, I am willing to provide a detailed explanation of how I arrived at my solutions: my thinking process, test cases I used, how I debugged my code during the contest, etc. I followed the contest conditions: no outside assistance, no AI-help, no code sharing, and I submitted within time. If there are any particular concerns or similarities flagged by the system, I respectfully believe they can be explained by coincidence or by common patterns rather than misconduct. I respectfully request you to review my submission history and, if possible, restore my contest participation and rating/standing for Contest 2169. Thank you very much for your time and understanding.

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6 months ago, hide # |
Rev. 2  
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Hello, I received a similarity warning for my solution in this round. I did not share my code with anyone or copy from anyone. It is possible that I once used an online IDE that unintentionally made my code public. I understand this is against the rules even if unintentional, and I sincerely apologize. I will make sure this never happens again. Thank you for checking.

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5 months ago, hide # |
 
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Hello. I received a system message saying that my submission for problem 2169B (submission ID 348926906) significantly coincides with another user’s solution. I want to clarify that I solved the problem independently. My approach: I modelled the string as a directed graph, where each index has edges depending on <, >, or — (or other characters meaning both directions). Moving outside the string is treated as reaching a special boundary node. 1. I first detect cycles using an iterative DFS to ensure that no infinite loop exists. 2. If a cycle is found, the answer is -1. 3. Otherwise, I compute the maximum length path to the boundary using DFS + memoization. 4. The final result is the maximum of these lengths. This approach is a natural and common model for this problem since it essentially reduces to: build transitions, detect cycles, compute longest path in a DAG.

Implementation details proving independence: I used a sentinel node sh = n to represent out-of-bounds transitions. I implemented DFS iteratively (vector<pair<int, int>> stk) instead of recursion to avoid stack overflow. For DP, I used an explicit stack with (node, expanded) pairs to simulate post-order traversal. I used INT_MIN/4 as a “NEG” value in the DP memo array to avoid overflow. These patterns reflect my usual coding style and appear in my editor’s local history.

Why similarity may appear: The structure of the problem strongly guides solvers toward the same transitions + cycle detection + DP solution. Many correct solutions may therefore look similar in logic and structure even if independently written.

I can provide evidence if required:local file timestamps, IDE local history showing how I wrote the code, git logs (if needed), screenshots of my environment at submission time.

I did not share my code with anyone, nor used any public code-sharing service. Please let me know if you require further proof. I am willing to cooperate fully. Thank you.

—aashiumrao