atcoder_official's blog

By atcoder_official, history, 3 weeks ago, In English

We will hold UNIQUE VISION Programming Contest 2024 Summer (AtCoder Beginner Contest 359).

We are looking forward to your participation!

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3 weeks ago, # |
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atcoder is top 8 contributer now!

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After many many exams in the school, many many students begin their AtCoder tours again :)

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Idk how I passed C after 4 WAs, just did some +/- math. Idea was to move diagonally as much as possible and align the x coordinate or y coordinate with the target node.

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3 weeks ago, # |
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what the fck was C

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    3 weeks ago, # ^ |
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    Idk man, I just figured some +/- math after moving diagonally as much as possible and aligning resultant x or y with target node. After that it was tricky to move in the same direction where I did some guessing.

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    3 weeks ago, # ^ |
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    I felt like leaving the contest .

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3 weeks ago, # |
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Is there a simpler solution to G than Small to Large merging? Or did people just manage to implement that approach in like 5 mins @_@

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    3 weeks ago, # ^ |
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    small to large merging is simple ??

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    3 weeks ago, # ^ |
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    It's a classic euler tour + monostack problem.

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    3 weeks ago, # ^ |
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    There are indeed simpler solutions, but small to large merging is also simple enough. It's only about 30 lines and 5 mins or so is enough to implement it (my solution in contest was small to large merging).

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      3 weeks ago, # ^ |
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      Could you roughly explain your approach using small-to-large merging?

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can someone please explain the idea behind the 3rd question import math a,b=map(int,input().split()) c,d=map(int,input().split()) t=math.ceil(1.5*abs(b-d))

if t<=abs(a-c) and t!=0: print(int(abs(a-c) -1.5*abs(b-d))//2 + abs(b-d)+1) else: a=abs(b-d) print(a) please explain me why my code is incorrect please

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    3 weeks ago, # ^ |
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    you can try to move the initial and the end points to adjacent ones (where (x + y) mod 2 = 0) such that each one is still in its tile and then draft some examples

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My solution for problem-D is to store last K characters as state. AC

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3 weeks ago, # |
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Great contest. D was a really good problem, wish I would've solved E a bit quicker though. Also, How to do C?

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    3 weeks ago, # ^ |
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    Idea is to move diagonally as much as possible and then align target with the resultant x or y coordinate. Tricky part was to move in the same direction in x. In y its just that difference of y coordinates as height of tiles are 1.

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how can we get atcoder contest testcase at which our solution got failed...

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Anybody who found C,D tougher than E,F or just me?

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Anyone can give hints on how to solve D?.

My approach to count all the possibilities such there is atleast 1 subarray of K length that is a palindrome and then the subtract that from 2^q and get the answer, but I couldn't calculate it properly(due to overcounting).

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    3 weeks ago, # ^ |
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    bitmask dp

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    3 weeks ago, # ^ |
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    Let $$$dp[i][m]$$$ (for $$$k - 1 \leq i \lt n$$$) represent the number of ways of constructing a good prefix of the first $$$i$$$ characters ending with mask $$$m$$$.

    For $$$i = k - 1$$$, this is fairly direct, check all $$$2^k$$$ possible masks and see if they work (are possible and don't contain any palindromes). If they do, set $$$dp[k - 1][m] = 1$$$, otherwise set it to $$$0$$$.

    Now notice that the masks for position $$$i$$$ and $$$i - 1$$$ share $$$k - 1$$$ characters. So for any $$$i \geq k$$$, we can check each of the $$$2^k$$$ masks again. If a mask $$$m$$$ is valid, we simply calculate $$$dp[i][m] = dp[i - 1][m'] + dp[i - 1][m' + 1]$$$ where $$$m'$$$ is $$$m$$$ with its highest ($$$(k - 1)$$$-th) bit disabled.

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3 weeks ago, # |
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I think problem B's statement could have been made a lot easier to understand.

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My code failed for last three testcases in C.Can someone point out where the mistake lies?

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"Can anyone explain C, please? I should have at least tried d, as I've spent so much time with C, actually, not so much time but significant time. I'm feeling bad about it now."

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3 weeks ago, # |
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D is good.

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I used small to large in G. But I can't understand what's wrong in my solution. mp[v][c] contains two numbers: the number of vertices of color c in v's subtree and the total length of paths to them.

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UPD: submission

UPD2: I found issue, AC

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If C has a million haters I'm one of them. If C has a hundred haters I'm one of them. If C has one hater it's me. If C has zero haters I have died. If the world is against C I am with the world, if the world is for C I am against the world.

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    3 weeks ago, # ^ |
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    D E easier than C

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    3 weeks ago, # ^ |
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    Tbh C was cute problem. I didn't like it either, but it's really simple and solution is pretty concise. I feel like CP turned into mindlessly rewriting same algos over and over, especially on AtCoder ABC. Problems like this humiliate you in a good way.

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      3 weeks ago, # ^ |
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      I was helllbent on solving C, finally did and missed on other problems D,E,...

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      3 weeks ago, # ^ |
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      I don't particularly enjoy writing math casework

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    3 weeks ago, # ^ |
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    I also don't like problems like this one. But since ABC is supposed to be educational, I try to make use of such problems. For example, I've learned the following lessons (in this particular order):

    1. Skip nasty non-standard grid problems whenever it's possible. Especially if there are problems with more points to get.

    2. Try to find out "simple observation" (as they will call it in editorial for sure) before considering shit tons of edge cases.

    3. Test your solution like crazy. After the first WA, write stress test immediately, don't wait for luck.

    4. Solving more and more similar problems make your more resistant and prepared to them.

    5. Competitive programming is 100% pain.

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    3 weeks ago, # ^ |
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    If C has n haters ($$$ 3 \le n $$$) I'm one of them. If C has two haters that's you and me. If C has one hater I have AFOed. If the world is for C, I'm with you.

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3 weeks ago, # |
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E was stack and $$$\text{O(N)}$$$ ? (Asking cuz constraints had 2e5)

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So typical problem G. Maybe it will be better to swap some orders of the problem.

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Could you guys just STOP making "greedy in grid" problems such as C plz? :(

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how can we get atcoder contest testcase at which our solution got failed...

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https://www.luogu.com.cn/problem/P7840

this is the original problem of F?

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    3 weeks ago, # ^ |
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    Why ABC usually has an original problem?

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    3 weeks ago, # ^ |
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    My code which get AC in AT also get AC in LG!

    But I suppose it wasn't on purpose, as there weren't many easy ideas:(

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    3 weeks ago, # ^ |
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    But I think ABC is a good way to let us find the same problem so that get AC.

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      3 weeks ago, # ^ |
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      so that get AC is wrong in grammar,you should modify it to be "so that you can get AC on the original problem."

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This problem is same as F!

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C was exactly same as https://codeforces.com/contest/1421/problem/D. Just consider the right point of tiles (moving between left and right didn't encounter any cost), and those right points forms the hexagon.

It can be solved via linear algebra, no need to go into greedy

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3 weeks ago, # |
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Problem F has appeared here previously : https://www.luogu.com.cn/problem/P7840

However, Luogu is a website that currently mainly used in China. So I think it is a coincidence. I only point out for notice.

Here is the translation by Copilot:

  • There are $$$n$$$ servers.
  • Each server has a busyness level denoted by $$$v_i$$$.
  • The servers are connected using $$$n−1$$$ network links.
  • Each server has a connection count $$$d_i$$$.

The total running time of the server network is given by the expression:

$$$ \sum_{i = 1}^n d_i^2 v_i $$$

The goal is to minimize this value.

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    3 weeks ago, # ^ |
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    I think so.

    I wonder who finds this original problem?

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3 weeks ago, # |
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Didn't Atcoder check for if there have been the same problem???

This problem from Luogu same as today's G.

And this problem is from the year 2021.

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    3 weeks ago, # ^ |
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    It is a Chinese website and it only support Chinese... It is extremely difficult to check EVERY CP website and search whether there is a same problem. So It can be only coincidence. BTW it didn't cause any significant effect, because it is just an ABC contest.

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      3 weeks ago, # ^ |
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      They do nothing wrong. Try not to be too aggressive.

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        2 weeks ago, # ^ |
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        Is it hard to check weight? Inexplicably! Only a fool thinks so. The questions are exactly the same, and luogu is a great site for brushing up on questions, and the bottom line as a question writer is to not come up with exactly the same questions. Even though the questions are in Chinese, can't you learn Chinese? You can't read Chinese, can you? And ziyistudy is stating facts and you're defending a shit group of question writers. I think you are very wrong.

        Translated with www.DeepL.com/Translator (free version)

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    3 weeks ago, # ^ |
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    I don't think the testers can find this problem. It's from China.

    However, the impact is not significant, as even Chinese people find it difficult to detect.

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      find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect. find it easy to detect.

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Why inclusive-exclusive algorithm doesn't work in problem D?

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    3 weeks ago, # ^ |
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    I'm stupid... How dare me apply inclusive-exclusive algorithm for n-k elements!!!! I thought n-k<=10....

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Top 10 contribution!

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I wonder how to prove that greedy algorithm is correct for problem F. Would anyone like to share your ideas? Thanks a lot.

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I see some solutions using Small to Large technique, so I'd like to share 5 problems (in increasing order of difficulty) for someone who is not familiar with this topic and want to learn it.

Plain Small to Large

  1. Distinct Colors
  2. Confuzzle
  3. It is a Tree : Subtask 2

Small to Large + Advanced DP

  1. Count Paths
  2. Leaf Color

For problems from BOJ, either use Google Translate or add the problem to Vjudge and use the inbuilt translator option.

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Can anyone explain Problem G ?