rui_er's blog

By rui_er, history, 3 weeks ago, In English

Hello, CodeForces.

I'm writing this blog entry to petition to ban FerjaniSassi for their mass harassment and spam blogs.

I was pinged by them for 4 times in their spam blogs, some of which were removed (probably by CodeForces).

After I checked their blog, I was shocked that they pinged thousands of users rated from Expert to LGM in a single blog entry! There are 2 accessible blog entries with mass harassment, and I think there must be more similar blog entries that were removed (because I was pinged for 4 times). Also, their other blog entries didn't contain any meaningful information. These blog entries only contained paragraphs in invalid languages or pictures with unknown meaning.

I believe that mass harassment and spam blog entries shouldn't be allowed in CodeForces. If they also harassed you, or you agree with my opinion, please upvote this blog entry to let me know.

I'd like to ping MikeMirzayanov to check this.

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By rui_er, history, 7 weeks ago, In English

Hello, CodeForces.

As I can't find any discussion thread in AtCoder, and the announcements of AtCoder contests are also posted here, I decided to post this blog in CodeForces.

Last time (ABC355), I reported a suspicious participant toyuzuko who solved problem ABCD in 51 seconds. They also did the similar thing in ABC354.

This time (ABC356), there are much more suspicious participants.

The average first-AC time (in seconds) of ABC347~ABC353 is in the following table:

Contest A B C D E F G
ABC347 21 45 93 244 177 300 824
ABC348 15 37 72 302 236 495 430
ABC349 23 67 54 116 598 139 634
ABC350 26 47 102 74 240 489 335
ABC351 22 23 106 352 199 122 902
ABC352 27 43 49 90 196 989 188
ABC353 24 79 172 88 206 1161 313
(minimum) 15 23 49 74 177 122 188
(prefix sum) 15 38 87 161 338 460 648

Note that to get first-AC in a problem doesn't require you to solve all the problems before it. You can just skip to the problem you want to get first-AC and manage to solve it at once. So, the least reasonable time to solve a subset of problems is at least the sum of the first-ACs.

To make sure there are fewer exceptions, I will call all the participants who solved any subset of problems faster than the minimum expected time "suspicious".

Now, let's take a look at (some of) the suspicious participants who solved at least 4 problems at last:

Some other participants seem to be suspicious, too. They solved a subset of problems just 10 seconds slower than the sum of the minimum first-AC time in the above table. I think it's impossible to have all the easy problems in a single round, but I didn't list them here, just to make sure there are fewer exceptions.

I guess they probably used some AI to generate the code for them after seeing what happened in ABC355. If my guess is true, I'm impressed with the ability of AI. I'm also worried that more and more people will use AI to generate the code and solve easy problems in the contest. If everyone can solve ABCD by simply copy-pasting AI's code, what's the point of having AtCoder Beginner Contests? Why not just delete ABCD and add some new problems to make an AtCoder Regular Contest?

You can leave your opinion in the comments.

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By rui_er, history, 3 months ago, In English

1972A - Contest Proposal

Hint 1
Tutorial
Solution

1972B - Coin Games

Hint 1
Hint 2
Tutorial
Solution

1967A - Permutation Counting

Hint 1
Hint 2
Hint 3
Tutorial
Solution

1967B1 - Reverse Card (Easy Version)

Hint 1
Hint 2
Tutorial
Solution

1967B2 - Reverse Card (Hard Version)

Hint 1
Hint 2
Hint 3
Tutorial
Solution

1967C - Fenwick Tree

Hint 1
Hint 2
Tutorial
Solution

1967D - Long Way to be Non-decreasing

Hint 1
Hint 2
Hint 3
Tutorial
Solution

1967E1 - Again Counting Arrays (Easy Version)

Hint 1
Hint 2
Hint 3
Tutorial
Solution

1967E2 - Again Counting Arrays (Hard Version)

Thanks A_G for discovering that E2 is possible!

Hint 1
Hint 2
Tutorial
Solution

1967F - Next and Prev

Hint 1
Hint 2
Hint 3
Tutorial
Solution

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By rui_er, history, 3 months ago, In English

久别无恙, Codeforces!

We are glad to invite you to take part in Codeforces Round 942 (Div. 1) and Codeforces Round 942 (Div. 2), which will start on Apr/30/2024 17:35 (Moscow time). You will be given 6 problems and 2 hours and 30 minutes to solve them in both divisions. Some problems will be divided into two subtasks.

The problems were authored and prepared by N_z__, wyrqwq, yinhy09 and me rui_er.

We would like to thank:

Score distribution:

  • Div. 2: 500 — 750 — 1500 — (1000 + 1500) — 2500 — 3500
  • Div. 1: 750 — (500 + 750) — 1250 — 1750 — (1750 + 1000) — 3500

We hope you'll like the problemset!

UPD: Congrats to the winners!

Div. 2:

  1. 3_m
  2. RinaRin
  3. eightniko
  4. liumohan
  5. golomb

Div. 1:

  1. tourist
  2. jiangly
  3. jqdai0815
  4. gamegame
  5. Kevin114514

Editorial is out.

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By rui_er, history, 15 months ago, In English

1797A - Li Hua and Maze

Tutorial
Solution (rui_er)

1797B - Li Hua and Pattern

Tutorial
Solution (rui_er)

1797C - Li Hua and Chess

Tutorial
Solution (rui_er)

1797D - Li Hua and Tree

Tutorial
Solution (rui_er)

1797E - Li Hua and Array

Tutorial
Solution (rui_er)

1797F - Li Hua and Path

Tutorial
Solution (Celtic, centroid decomposition)
Solution (rui_er, reconstruction trees)

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By rui_er, history, 16 months ago, In English

Hello, Codeforces!

AC-Automation and I rui_er are glad to invite you to participate in Codeforces Round 864 (Div. 2), which will be held on Apr/08/2023 17:05 (Moscow time). Please note the unusual start time.

This round is rated for the participants with ratings strictly lower than 2100. You will be given 6 problems and 2 hours to solve them. All the problems are prepared by AC-Automation and me.

We would like to thank:

There will be a finite number of interactive problems, so please see the guide of interactive problems if you are not familiar with it.

The main character of the problems is Li Hua.

Who is Li Hua?

Statements and editorials will be available in Chinese (Simplified) after the contest.

This is our first round. We've tried our best to make the round enjoyable. We are looking forward to your participation and hope you gain a non-negative delta in this round.

UPD1: Score distribution: 500 — 1000 — 1500 — 1750 — 2250 — 3000.

UPD2: Editorial is out.

UPD3: Statement in Chinese and Editorial in Chinese are out.

UPD4: Congrats to the winners!

Div.2:

  1. syf2008
  2. elizazh
  3. zhouzizhe
  4. psz1
  5. wnmrmr

Div.1 + Div.2:

  1. jiangly
  2. heno239
  3. SSRS_
  4. BurnedChicken
  5. Sugar_fan

Also congrats to the only div.2 participant who solves problem F: reborn2023!

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