Theo830's blog

By Theo830, 3 years ago, In English

1594A - Consecutive Sum Riddle

Hint
Solution
Code (C++)

1594B - Special Numbers

Hint
Solution
Code (C++)

1594C - Make Them Equal

Hint
Solution
Code (C++)

1594D - The Number of Imposters

Hint
Solution
Code 1(C++)
Code 2(C++)

1594E1 - Rubik's Cube Coloring (easy version)

Hint
Solution
Code (C++)

1594E2 - Rubik's Cube Coloring (hard version)

Hint
Solution
Code (C++)

1594F - Ideal Farm

Solution
Code (C++)
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3 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Wow the editorial even before the system tests are finished!

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3 years ago, # |
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Thx for the fast editorial! :)

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3 years ago, # |
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Thanks for fast editorial

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3 years ago, # |
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Thank you for the contest

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3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Great problem set! Thank you

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3 years ago, # |
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What is the intuition behind B?

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    3 years ago, # ^ |
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    for all $$$n \geq 2$$$ and $$$k \geq 1$$$, $$$n^k > \Sigma_{i=0}^{k-1}n^i$$$

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    3 years ago, # ^ |
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    Special number is n base number with only ones. So now we have to figure out how to calculate the k-th one And because it is just a matter of play between ones and zeroes, you are like in a binary number, so the k-th number will be the n-th base number with ones on the same spot as k

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    3 years ago, # ^ |
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    The numbers that we need to form should have distinct powers and their coefficient should be either 1 or 0. For example if we take n = 4 and k = 17 then 17 = (10) base 4. So this would give an intuition that we need to do something with binary strings or bianry representation of a number K as we need coefficient as 0 or 1 of the powers. Now Lets see what we have to do K = 1 in binary = 1 K = 2 in binary = 10 K = 3 in binary = 11 K = 4 in binary = 100 and so on Now lets see how our answer comes out, As we need to find the Kth special number for base n so suppose that above binary strings were formed with base n and then convert them into their decimal equivalent respectively. For example from test cases where n = 3 and k = 4 so now binary representaion of k is 100. Just take this in base n = 3 so it comes out to be 1*3^2 + 0*3^1 + 0*3^0 = 9 and our answer. Similarly take n = 2 and k = 12. So 12 = 1100 in binary. As our base n is also 2 only answer again converts back to 12. You can further take more examples.

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3 years ago, # |
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Oh , that was realy fast!

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3 years ago, # |
  Vote: I like it -181 Vote: I do not like it

Testers for this round be like

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    3 years ago, # ^ |
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    This is really rude. Testers did their best and gave invaluable feedback. No-one can know every problem that has been created. We are sorry for the situation but testers are not to blame.

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3 years ago, # |
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Nice contest. Thank you so much!!!!

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3 years ago, # |
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That was really nice contest! Great statements, beautiful solution and very fast edutorial! (even faster than sys tests). I really enjoyed these 2h 15m in this contest, thank you!

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3 years ago, # |
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E2 is pretty interesting I think.

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3 years ago, # |
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Thanks for the fast editorial!

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3 years ago, # |
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Nice round.

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3 years ago, # |
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excellent contest with an interesting problemset , i got very close to solving 3 problems in a div2 for the first time ever , hopefully gonna do it next time :D

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    3 years ago, # ^ |
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    same for me too! i was close to solving C and A,B was very interesting :D

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3 years ago, # |
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D was completely copied from INOI 2021... how can you blatantly copy a problem

https://www.codechef.com/INOIPRAC/problems/AMONGUS2

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    3 years ago, # ^ |
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    lmao

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    3 years ago, # ^ |
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    how did they do this? 1) You highly underestimate the popularity of among us 2) As pointed by someone in the contest announcement comments that it is a little bit trivial for experienced coders to come up with that idea for the problem... check that comment I am lazy to link it.

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      3 years ago, # ^ |
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      yeah but they can atleast put some effort..Like change some thing introduce some different parameter exactly copying a statement is too much

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        3 years ago, # ^ |
          Vote: I like it +35 Vote: I do not like it

        We did not copy anything and would never ever do that as it ruins the whole point of problemsetting for both authors and contestants. This was an unfortunate coincidence and we are very sorry for it, we will be taking every measure to avoid it in the future.

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    3 years ago, # ^ |
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    SUS

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    3 years ago, # ^ |
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    why y'all downnvoting its not like i am disrespecting ..I am just stating an obvious fact..Really cant get this community sometimes

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3 years ago, # |
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Can someone please tell me why my solution for E gives wrong answer submission.

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    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Your val variable which stores the total number of nodes has been stored modulo 1e9 + 7. And then you are using val as a power. If you are calculating a^b%MOD then b has to be stored not modulo MOD but rather a different number. In this case I think its 1e9+6 so you had to store val modulo 1e9 + 6.

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    3 years ago, # ^ |
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    Your calculation of $$$val$$$ is wrong.

    Fermat's little theorem says $$$a^{p-1} \equiv 1 \bmod p$$$, so you have to calculate the exponent modulo $$$p-1$$$ (in this case, $$$10^9 + 6$$$).

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3 years ago, # |
Rev. 3   Vote: I like it +16 Vote: I do not like it

Better approach for F, for case s > k:

For each block of size k,

Put, (k-1) continuous ones, and then put (k+1) to avoid subarray with sum k. (1, 1.. k-1 times) (k+1)

This is the basic case, when the farm isn't lucky, all other cases of the farm being unlucky will require higher value of s.

Sum left, after putting all ones = s - n

Extra Sum Required = (n / k) * k (for basic case, described above)

if sum_left < extra_sum, then "YES"

else, "NO"

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    3 years ago, # ^ |
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    I had the same idea,but I failed to perfectly prove "when the farm isn't lucky, all other cases of the farm being unlucky will require higher value of s."

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    3 years ago, # ^ |
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    Could you plz tell me how to prove it?thx

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      3 years ago, # ^ |
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      Firstly you can porve that if only use k blocks,the minimum n is $$$2\times k$$$.

      Then you will find that the way to construct exactly fits the conclusion above,if only use k blocks,the minimum n is $$$2\times k$$$ first.


      The above conclusion can be proved by using Pigeonhole principle.

      If there is a legal way that has n less than $$$2\times k$$$,it should satisfy that for each $$$pre_i\bmod k$$$ must be distinct,because if $$$pre_i\equiv pre_j\pmod k$$$,the subarray $$$[i+1,j]$$$ will be equal to k,($$$pre_i<2k$$$).

      It is impossible because we have $$$n+1$$$ $$$b_i$$$,and $$$n$$$ $$$b_i\bmod i$$$.

      Then we has already proved the conclusion.

      Sorry,I'm not certain about whether my proof is correct.

      Hope it can help you :)

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3 years ago, # |
  Vote: I like it +10 Vote: I do not like it

Speedforces, and fast editorial.

I missed " if(s==k){ cout << "YES\n"; continue; } " and got F wrong. :(

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    2 years ago, # ^ |
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    Can you explain me problem please. I think , I am confuse about what is problem asking.

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3 years ago, # |
Rev. 3   Vote: I like it +17 Vote: I do not like it

For problem $$$C$$$ it's just enough to check if any of the chars beetwen $$$[n/2+1, n]$$$ (1-indexing) is equal to $$$c$$$ , if it's then the answer is 1. $$$Proof$$$ : Any number between $$$[1, x - 1]$$$ and $$$[x+1, x*2-1]$$$ isn't divisible by x, so every index that isn't $$$x$$$ ,will be changeable. Time complexity — $$$O(N)$$$. My submission — 131189321. Btw, nice contest, thank u!

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    3 years ago, # ^ |
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    Thank you for this, was trying to understand this in tourist's code lol

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    3 years ago, # ^ |
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    You need to say why it's enough to check $$$[\frac{n}{2}+1, n]$$$, because if $$$x$$$ in $$$[1, \frac{n}{2}]$$$, $$$2x$$$ is in the second half, so in order to use $$$x$$$ it must be that the character at $$$2x$$$ is already correct. So we can use the second half without loss of generality.

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      3 years ago, # ^ |
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      Your are right, in order to use x in the $$$[1, n / 2]$$$ we need to check $$$2x,3x..$$$ as well, but instead of checking $$$x,2x,3x...$$$ we just need a maximum $$$yx$$$ such that $$$yx <= n$$$, and if we mark that $$$yx$$$ then every number $$$x,2x,3x,4x..<yx$$$ will be changeable because $$$u$$$ $$$mod$$$ $$$p$$$ $$$!=$$$ $$$0$$$ when $$$u < p$$$. That's why my solution uses number $$$[n/2+1, n]$$$ because there is $$$NOT$$$ any number $$$x$$$ in that range such that $$$2x <= n$$$.

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3 years ago, # |
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Nice problems. Thank you!

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3 years ago, # |
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3 years ago, # |
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Thanks for an amazing contest.I really like problem D and editorial is very good)

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3 years ago, # |
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Video Editorial for Problem D explaining the 2 coloring strategy for bipartite graphs

I solved the original problem in contest so I explain my exact thought process which might help some of you.

Note that I didn't do this contest because I saw D before starting and didn't want to take an unfair advantage.

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    3 years ago, # ^ |
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    I know, You are a gem! Keep posting such valuable videos. You will surely have a great future ahead. I follow your videos and they are very helpful. Don't mind about the downvoters, such people always exists.

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    3 years ago, # ^ |
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    ak2006 Nice editorial man, keep up the good work. I've seen your content as well it's really educational.

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3 years ago, # |
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Can someone please explain E1?

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    3 years ago, # ^ |
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    For the root node you have 6 colours to choose from, and for the rest you have 4. Then, the answer must be 6 * (4 ^ ((2 ^ k) — 2))

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      3 years ago, # ^ |
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      I did with dp i can have ans if n goes till 1e8 its sometimes very interesting to know that the solution was very diffrent[submission:131209883]

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3 years ago, # |
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can someone explain me problem B?

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    3 years ago, # ^ |
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    Basically, just think like this: For a given n, we need to find the kth integer in base n. Why so? If we need to get to that kth integer, we need to go through all the possible sum of powers of n, which happens to be the process of finding kth integer in base n.

    To understand it clearly, take n = 2.

    And for the solution, look for the set bits of k. And add all the powers of n to the set bit positions.

    Please ask if you need more elaboration on this.

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      3 years ago, # ^ |
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      to get the kth number in base of n , we need to add to sum of powers in the indexes of set bits , for example in case of n=2 , we are getting all natural numbers , but these numbers have a base of 10 , i am confused with this base thing

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        3 years ago, # ^ |
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        For n=2, we are looking at the kth number in base 2, which goes from 1, 10, 11, 100, 101 and so on.

        For n=3, we are looking at it in base 3, so 1, 2, 10, 11, 12 and so on. But:

        the question said that special numbers are sum of different powers, so the base can only contain 0s and 1s, so we will ignore 2, 12, 20 and so on.

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    3 years ago, # ^ |
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    just find binary representation of k and then add n^i where i is all set bits of k.

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3 years ago, # |
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How can u think for B . is it number theory or something else ? And how can i be able to solve these type of questions? Pls somebody help me

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    3 years ago, # ^ |
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    First, let's see what the array is if n=2.

    We already know that every integer can be represented via a sum of powers of two (and no two powers are equal). This happens because.. well.. every number has a distinct form in base-2, and the base-2 writing actually indicates you on what powers of two to add.

    Now, let's see for n=5 for example, what the array will be

    1 = 1(5) 5 = 10(5) 6 = 11(5) 25 = 100(5) 26 = 101(5)

    And so on.

    Observe that the base-5 writing of these numbers looks very similar to the base-2 writing of 1,2,3,4,5... So, we can state the following:

    If we know the base-2 writing of K, then we can just change the base to n (we keep the number as it was). And from here, I think it's pretty straightforward.

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    3 years ago, # ^ |
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    It's very basic properties of positional notation (positional numeral system). Read about it.

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3 years ago, # |
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Nice problems. Thank you!

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3 years ago, # |
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Excellent contest, System testing was also awesome :D

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3 years ago, # |
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Maybe 1594C has an $$$O(|s|)$$$ method?

Here is my solution.

First, if we have a minium i such that $$$s_i,s_{i+1},...,s_n \neq c$$$ and the answer is $$$1$$$ operation with a certain $$$x$$$, it is obvious that $$$s_x = c$$$ and $$$\forall k \in N_+ \wedge kx \leq n, kx < i$$$.

If $$$i = 1$$$, we can't find that $$$x$$$.

If $$$i \neq 1$$$, let $$$a = i - 1$$$.

We can prove that if $$$a$$$ doesn't match the conditions above, we can't find that $$$x$$$. If $$$a$$$ doesn't match the conditions above and there exists $$$x$$$ such that $$$x < a$$$, $$$i \leq 2a \leq n$$$ and $$$\exists k \in N_+, a < i \leq kx \leq a + x < 2a \leq n$$$, so $$$x$$$ doesn't match the conditions above.

And if and only if $$$2a > n$$$, $$$a$$$ can be the $$$x$$$, because $$$2a > i$$$, $$$\forall i \in N_+ \wedge i < a, i$$$ is not divisible by $$$a$$$ and only at this time $$$\forall k \in N_+ \wedge k > 2, ka > n$$$.

Then we can use $$$O(|s|)$$$ to find this $$$a$$$ and output it, solving the situation when the answer is $$$1$$$ operation with a certain $$$x$$$.

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3 years ago, # |
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Great Editorial!!

One can check this youtube channel for video editorials: Link

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3 years ago, # |
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B harder than E1 !

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3 years ago, # |
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regarding C — "Make Equal" :

Can someone please tell me where my code gives the wrong output? The wrong answer is for test case 74 of preset 2 but I'm not able to see what it is. I've been trying to find my mistake for more than an hour.

idea — I tried to check if there is any index > n/2(n/2 + 1 for even numbers ) that has the correct character. If yes, then the required number of x is one. else 2.

Link to the submission

Code
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    3 years ago, # ^ |
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    you can link a submission or put the code in a spoiler

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3 years ago, # |
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Contest was actually good, I coded F in O(min(s — n, n) / k), it passed pretests, then I changed in to O(1) but after the round it still works. Strange tests to be honest. Here is my solution https://mirror.codeforces.com/contest/1594/submission/131243246

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    3 years ago, # ^ |
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    Actually yes, your code gives TLE on this test

    Test
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3 years ago, # |
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I have a solution in problem C that runs in O(|s|) time complexity Firstly, I'll check if all characters in the string are equal to C and if so I will print 0(no need to do any operations).

Next, I check if I can use 1 operation only and construct a good string at the end by finding an index such that (s[i] == c) and (i * 2 > n) and doing 1 operation using this index i. Let's divide proving the second step into two parts: Proving that this operation is enough & Proving that if such an index doesn't exist then simply we can't change the whole string to be equal to c in 1 operation. 0) Choosing index i means that all indices <i are changed to C(since none of them are divisible by i) and since 2 * i>n then all indices between i+1 and n are changed to C too.

1) The chosen index must have s[this_Index]=c because we are always not changing any of its multiples including itself and building on this fact we need to choose an index i such that all characters on indices that are multiples of i(if any exists) are equal to c. The thing is that the (largest multiple of i that is <=n) *2 must always be >n hence that an index such that s[this_index] is equal to c and this_index * 2 >n must exist or there will be no way to do it in 1 operation.

Finally, if I didn't find such i that can fix the string in 1 operation then I will fix the string in 2 operations by making x = n and this will make all of s[i] = c for 1<=i<n and then use x = n-1 to make s[n] = c

My solution: https://mirror.codeforces.com/contest/1594/submission/131192419

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3 years ago, # |
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Easier solution for 1594F - Ideal Farm.

for (int q = 0; q < Q; q++) {
	ll S = nl();
	ll N = nl();
	ll K = nl();

	if (S==K)
		cout << "YES\n";
	else if (S < K || S >= N+(N/K)*K)
		cout << "NO\n";
	else
		cout << "YES\n";
}
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    3 years ago, # ^ |
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    lol I sent the same, now I dont care about the right solution, I am just interested in tests' weakness

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3 years ago, # |
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I think it's just simply one of the most interesting contest i've written here.

Thanks!

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3 years ago, # |
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Can you please explain the end of the tutorial of F? I. e.:

"Let m be the maximum number of elements that we can take.

We go through the last k elements ([s−k+1,s]) and we count the number of elements that have the same modulo k.

For each element in this range, if there are odd elements that have the same modulo, we can't take all of them because for every element x that we add in pre that we also add x+k to b. Thus one element would have a x+k out of range.

Therefore we count all the elements that have odd elements with the same modulo k and subtract them from s+k to find m."

I can't understand literally anything from here.

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    3 years ago, # ^ |
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    $$$m = $$$ the maximum number of elements that we can take among the $$$[1,s + k]$$$. (We want to make $$$pre$$$ of size $$$n$$$ and $$$b$$$ of size $$$n+1$$$).

    We only need to build $$$pre$$$ but there should not same elements in $$$pre$$$ and $$$b$$$.

    We go through the last $$$k$$$ elements ($$$[s−k+1,s]$$$) and we count the number of elements that have the same modulo $$$k$$$.

    For each element in this range, if there are odd elements that have the same modulo, we can't take all of them (and add them to $$$pre$$$) because for every element $$$x$$$ that we add in $$$pre$$$ that we also add $$$x+k$$$ to $$$b$$$. Thus one element would have a $$$x+k$$$ out of range ($$$x + k > s + k => x > s$$$ which is obviously wrong) .

    $$$pre: $$$

    ($$$[s−k+1,s]$$$)

    ($$$[s−3k+1,s-2k]$$$)

    $$$...$$$

    $$$b: $$$

    ($$$[s+1,s+k]$$$)

    ($$$[s−2k+1,s-k]$$$)

    $$$...$$$

    Therefore we count all the elements that have odd elements with the same modulo $$$k$$$ and subtract them from $$$s+k$$$ to find $$$m$$$.

    I hope that now it's clearer.

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    3 years ago, # ^ |
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    The solution means that we need to allocate the elements of $$$[1, s + k]$$$ into $$$2n + 1$$$ positions (some elements may not be allocated).

    Firstly we can observe that for every element $$$x$$$ assigned to pre there's a corresponding $$$x + k$$$ assigned to b, this means that the numbers with the same modulo $$$k$$$ must be allocated even number times. So we can divide all the numbers with the same modulo $$$k$$$ into the same group.

    Noticed that if there is an odd number of elements in a group, then the group cannot be fully allocated, so the total number of elements that can be allocated must be subtracted by one. It is easy to figure out how many groups of odd size we have, we can subtract the number of groups with odd size from $$$s + k$$$ finally we will get the number of elements that we can allocate.

    But note that $$$k$$$ must initially be assigned to b (because $$$b[0]=pre[0]+k=0+k=k$$$ ), so the number of groups in which module $$$k$$$ is equal to $$$0$$$ must be subtracted by $$$1$$$.

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      3 years ago, # ^ |
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      The last paragraph is wrong. When the modulo k is 0 then it's like we take the element 0 in pre and k at pre. Therefore, you again have to subtract 1 when there are odd numbers that have 0 modlulo k.

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        3 years ago, # ^ |
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        I just consider the elements in $$$[1,s+k]$$$, so the element $$$0$$$ doesn't count.

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3 years ago, # |
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in problem D i submit thiscode but it recieved wrong answer it use directed edges but when i convert edges to bidirectionl it get accepts codewhy this happens?

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    3 years ago, # ^ |
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    Bruh, how can you still be expert with that amount of solved problems lmffaaaoooo

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    3 years ago, # ^ |
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    I have similar query as well. Did you get to know the reason why this happened?
    Code with Directed Edges: Directed
    Code with Undirected Edges: Undirected

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      3 years ago, # ^ |
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      because if 2 says 3 is a liar and 1 says 3 is not a liar then see from first two possibilities are there for 2 and 3 they r 2 is liar while is not or 2 not liar and 3 is. Based on this we can get information about 1 since 1 says 3 is not a liar then if 3 is a liar then 1 is also (see the reverse edge) and if 3 is not a liar then 1 is also not. Thus if we have 2->3 and 1->3 this can be rewritten as 2->3 and 3->1 hence bidirectional

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        3 years ago, # ^ |
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        yeah, I got your point. Thanks for the explanation. Can you please further tell, why solving with a directed graph gives a wrong answer. I mean, if you have some counter case.

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3 years ago, # |
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My feedback -
I just read F and despite multiple readings couldn't figure out what exactly it means. I tried guessing but couldnt come up with something. I wasnt participating but just decided to look at F in middle of the contest, but would have submitted something on F if I had a soln.
- Animal pen doesn't make any sense. Something which animals eat could have been better.
- There is no clarification if there are total n pens or if there are n types of pens with infinite quantities of each type.
- What exactly do you mean by "empty pen"? You haven't defined what exactly is "empty pen"/"non-empty pen" is.
- "all animals in all pens". I tried reading it as "you have to distribute n pens among k animals such that ....." Couldn't come up with something to fill those .... here.

Although authors tried to give a formal definition? But a more formal definition would have been better. Formal definitions should be absolutely formal just like the definition after "The problem is the same as" in the editorial. Someone who just reads it should be able to grasp what is being asked and it shouldn't involve references to the story.

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    3 years ago, # ^ |
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    Did you perhaps misunderstand what pens are? [Pen](https://en.wikipedia.org/wiki/Pen_(enclosure))

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      3 years ago, # ^ |
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      Yeah. I did. Thanks.

      Sorry for my bad English :P

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      3 years ago, # ^ |
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      I've been living in the UK for three years and I've never heard the word 'pen' in this context. Anyone who knows that pen is a thing to write with should be careful about using a word with double meaning...

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        3 years ago, # ^ |
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        Then what is the word for pen which the UK citizens use? Animal pen is quite common usage.

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          3 years ago, # ^ |
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          I think they use the word "pen", but to use it, you must talk about animal pen, which nobody does among city citizens.

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3 years ago, # |
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Me last night on problem A:

Spoiler
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    3 years ago, # ^ |
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    What does it function means?

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      3 years ago, # ^ |
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      You mean "ctz"? "Count trailing zeros (in binary)", i.e., $$$\operatorname{ctz}(n) =$$$ largest integer $$$s$$$ such that $$$2^s$$$ divides $$$n$$$. C++ has it in supported architectures as __builtin_ctz, or there is an old algorithm using bitshifts and compares which is essentially hand-coded binary search.

      Or you can always fall back on the naive $$$O(\log n)$$$ approach.

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3 years ago, # |
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I thought problem E2 was really cool.

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    3 years ago, # ^ |
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    can u please explain me the approach i cannot solve it and also do not got it from editorial

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      3 years ago, # ^ |
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      The idea is that if we have a node $$$x$$$, whose color we arbitrarily fix, and nothing in the subtree of $$$x$$$ is fixed (in the sense that there are no restrictions on nodes below $$$x$$$), then we have $$$4^{sz - 1}$$$ ways to fill out the subtree. There are a lot of nodes of this type; in fact, 99.999% of the nodes are nodes of this type.

      There are also other types of nodes: nodes in which some child or child of a child, etc is fixed. These cases are a little bit harder and to do so, we can just memoize on its two left and right children. Fix the color on the left child, fix the color on the right child, and then take the sum of the products of sub[left] and sub[right].

      131252148

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3 years ago, # |
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A nice round, thanks for the fast editoria!!!

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    3 years ago, # ^ |
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    Tou xiang fei chang nice,gei wo zheng xiao le.

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3 years ago, # |
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Good thing problem B was from aime lol.i got positive delta.best round ever.easy google

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3 years ago, # |
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in problem E1, "a white node can not be neighboring with white AND yellow nodes", I belive the preposition used should be "or" instead of "and". I only read the formal statement, got confused, and ended up solving another problem for half an hour before realizing how dumb I am :(

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3 years ago, # |
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I think problem F's difficulty at most be *1800.

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    3 years ago, # ^ |
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    I still don't know solution. I didn't read editorial yet. Only for problems I solved.

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3 years ago, # |
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I think in problem C O(nlog(n)) can be reduced to O(n).

Notation: n=|s|; c=required char to be set
If all s[i]==c then ans=0
Else if at any of these positions (n/2 + 1,....,n), s[i]==c, then using x=i we can set all the rest positions in just 1 operation
Otherwise, all of these positions (n/2+1,....,n) will have s[i]!=c. So, they can't be changed in just one operation. So, answer will be 2 with x= n-1 , n

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3 years ago, # |
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why rating point for this contest is too low? 5 problem only for 7pts :'(

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3 years ago, # |
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can some one please tell me how to solve e2 i am unable to get it from editorial(my mistake not of editorial)

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    3 years ago, # ^ |
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    In short, whole idea is to forget about whole tree and find out how many ways to color nodes which are on paths from root to already-colored. Then, multiply by 4 to the corresponding power. And to find out first thing, make dp[v][c] which would tell how many correct ways to color vertex v into color c and its subtree in any colors (without taking into account nodes we're not interested).

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3 years ago, # |
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Can someone explain to me Question C?

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3 years ago, # |
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Can someone explain how to code problem E1 in java? I have understood the logic but I'm getting TLE error. Please help, thanks in advance.

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    3 years ago, # ^ |
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    You should use your own power function. You can see how I coded it in the editorial to implement it yourself.

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3 years ago, # |
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More contest at 18 05

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3 years ago, # |
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in problem F: Why are we going through the last k elements?

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    3 years ago, # ^ |
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    I have no idea what editorial is telling about but this should help:

    Our goal is clearly to construct (judge if this is possible) prefix-sum array such that it has length n + 1, first element is 0, last element is s, the array is strictly increasing and the array doesn't have x and x + k simultaneously for any x (then and only then we would have subarray with length exactly k).

    Instead we will try to construct longest possible such array (and if it's length would be > n + 1, we can easily "shrink" (decrease it's length) it to obtain construction of array of length exactly n + 1).

    Now for each rem = 0, 1 .. k-1 let's look how many elements at most of this (prefix) array can be equal to rem MOD k.

    Clearly for particular "rem" one of optimal constructions is rem, 2k + rem, 4K + rem and so on. So you only need to count how many numbers in interval [0, s] are equal to rem MODULO k. Suppose their count is x. Then you can take at most (x + 1) / 2 of them to the prefix array. Do this for every rem = 0,1 .. k-1 and you will know what is the greatest possible length of this prefix array. The only corner case is when s == k, because you are always forced to contain 0 and s in the prefix sum array as first and last elements.

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      3 years ago, # ^ |
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      Could you please explain why could we easily "shrink" this array?

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        3 years ago, # ^ |
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        Ok, I figured it out. The elements need to have the same rem, and their difference is already 2K. It would be just greater if we shrink.

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      2 years ago, # ^ |
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      probably >=n+1. Thanks for the explanantion.

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    3 years ago, # ^ |
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    Because if we don't put the last k elements in pre, b will not have the elements from s to s+k.

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3 years ago, # |
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Those two code are same.The only difference between them is variable type, int and long long int. But one got accepted, another got runtime error. For int type variable got AC, long long int got RE. why ?

AC code: 131311594 RE code: 131309828

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3 years ago, # |
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Finally become expert QWQ. Continue learning!

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3 years ago, # |
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Video Solutions

A B C

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3 years ago, # |
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bad editorial

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    3 years ago, # ^ |
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    Can you be more specific about what you didn't understand?

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3 years ago, # |
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Thanks for the contest. Nice problems and fast editorial!

Question: is it possible to solve B without using bitmasks?

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3 years ago, # |
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can anybody please explain why is my solution not working for problem d? solution link : https://mirror.codeforces.com/contest/1594/submission/131297133

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    3 years ago, # ^ |
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    I am also getting same verdict on testcase 2 line 40 :(

    Update: Try this case 4 3 1 2 imposter 3 4 crewmate 2 3 crewmate

    Your code will give -1. But actual ans is 3.

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3 years ago, # |
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Fun fact this is the third problem called Make Them Equal

1154B - Make Them Equal

1417D - Make Them Equal

1594C - Make Them Equal

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3 years ago, # |
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In problem D, What is the intuition behind connecting fake node with A and B nodes if person A said that B is a crewmate.

Also, how to do the problem with dp / dsu as it is there in one of the problem tags

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3 years ago, # |
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After hours of reading the Problem F editorial, I still can't understand the last part about finding M. If someone else has the same problem as me, this might help.

Suppose we have distributed the array A. Let's build a prefix sum Pre[i] = Pre[i-1] + a[i]. Pre[i] is distinct and increasing. If there are i, j such that Pre[i] = Pre[j] + k then there must exist a segment on A which it's sum equal to k.

Now we want to check if there is a way to build Pre[i] that does not exist Pre[i] = Pre[j] + k. Pre[i] is distinct and Pre[i] is a value in range [1...S]. If we chose Pre[i] = x then we can not chose x+k, if we don't chose x+k, we can chose x+2*k .

For every p = [0...k-1] (remainder when divided by k), we can take from S number x = t*k+p (t <= S-p/k). The maximum way to choose number x(s) is t=(0, 2, 4, 6,...). Except p=0, the best way to choose is t=(1, 3, 5, 7,..) since Pre[i] > 0. For every p, the maximum way to choose x (or t) is Tp.

Now, M = sum of all Tp (p=[0...k]). If M>=n then there is a way that does not exist Pre[i] = Pre[i]+k, the answer is NO, otherwise, the answer is YES.

You don't have to count Tp separately, use some modulus and divine operation and you can get M easily.

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3 years ago, # |
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Thank you for the contest

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2 years ago, # |
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Can anyone help me figure out why my solution is giving RTE in problem D? I am using DSU and small to large merging. 133162809

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2 years ago, # |
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https://mirror.codeforces.com/contest/1594/submission/133176237 why I am getting Wrong answer. which case i haven't considered.

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    It looks like you only use operations for $$$x = 2$$$ or $$$x = 3$$$. In this case, for example, you cannot change the $$$6$$$-th position in the string, because $$$6$$$ is divisible by both $$$2$$$ and $$$3$$$.

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4 months ago, # |
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Is there any prove for the problem B ,, Like a mathematical proof in book or something?

If you have one please leave it in a reply I will be thankful <3