lior5654's blog

By lior5654, 5 months ago,

Hello Codeforces! (Long Post!)

Introduction

As I've recently solved my first ever $\ge$ 3000 rated problem (3300) on Codeforces — Codeforces Round #773 (Div. 1) Problem E — Special Positions and the solution that I came up with is different than the one presented in the official editorial, I thought that it will be valuable to the community if I share my approach.

Note that problem solving is a complicated & creative process. In the editorial, I show clean steps to reach the final solution, but the jump between each step is not always trivial (and sometimes hard!). Reaching this solution involved a large amount of drawings, observations, calculations & mistakes. There's no magic trick (at least to my knowledge :) ).

Alright, enough introduction, let's get started!

(click "The Editorial" below this line in order to be able to view the editorial)

The Editorial

Final Notes

1. Thank you Mangooste for creating such a challenging & educational problem!

2. Friendly reminder to upvote all of Monogon's blog posts

3. gitgud

• +123

By lior5654, history, 15 months ago,

As Israeli contestants posted both the APIO 2019 Discussion Thread & the APIO 2020 Discussion Thread, I feel obligated to post this year's discussion thread :D

This year, APIO 2021 is hosted again by Indonesia. For most of the participating countries, APIO 2021 serves as one of the main "TST"s (Team Selection Tests) for IOI 2021 (including Israel). The competition's official website is located Here.

The competition is IOI style, 5 hours long. A maximum of 50 submissions is allowed for each problem. A participant's final score is the sum of the scores the participant got on each problem. A participant's score for a problem is the sum of the scores the participant gained in each subtask of the problem. A participant's score for a subtask is the maximum grade the participant's submissions achieved for that subtask.

APIO Problems are of high quality and are considered good practice for those who wish to participate in IOI.

This thread is intended for discussing results. Furthermore, after APIO 2021 Open Contest ends, this thread shall also be used for discussing the solutions to the problems given in the competition.

Note: Before jonathanirvings writes a comment like this yet again, please note that discussing solutions before APIO 2021 Open Contest ends is prohibited. However, discussing results should be allowed as the official window (based on the official website) has ended.

• +67

By lior5654, history, 15 months ago,

Hello Codeforces!

Today was Codeforces Round #720 (Div.2), in which I solved problem D — Nastia Plays with a Tree. As I find my solution different from the solutions I heard of after the contest, I will explain it in detail in this blog post. I will note that you still might find value in reading this if you solved the problem, as my proposed solution also maintains at every stage the graph being a forest (i.e I don't create any cycles).

The Solution

Solution By FiveSixFiveFour

This was very challenging to implement during the contest yet eventually I got AC 10 minutes before the end of the round. Thanks for the contest! :D

• +59

By lior5654, history, 17 months ago,

Dear codeforces community. Lately, I have started participating in some division 1 rounds, and I experience a weird issue. Suppose some Div1's problems are >= 2100 in difficulty. Then most purples after reading the problems choose to quit and not do the round. The purples that do choose to do the round, might not even solve the first problem, but the main issue is that a round is only rated for you if you submit. The latest codeforces div1 round (round #707) had about 1300 registered participants, but only ~700 submitted something, and only ~620 solved a problem. So solving problem A with bad timing causes you to be last place, despite doing better than half of the participants.

I suggest that a round should be rated for a user if he views one of the problems of the round as a registered participants. Was this suggested before? I think atcoder had some contests that worked that way but I am not sure.. Any way, the current system causes low div1's to simply quit, causing the people that do submit and succeed to lose massive amounts of rating.

I would appreciate discussing the issue in the blog's comment section.

Thanks, lior5654

• +187

By lior5654, history, 20 months ago,

The same announcement, Expert posted 6 hours before Master. Expert got -10, Master got +74.

35 hours ago:

https://codeforces.com/blog/entry/85354

41 hours ago (6 hours before the above post):

• +6

By lior5654, history, 20 months ago,

Congratulations Monogon for achieving the #1 spot in the top contributor leaderboards!

• +35

By lior5654, history, 20 months ago,

Opening this thread to discuss Romanian Masters of Informatics (RMI) 2020 Problems & Results.

Hope everybody had a great competition!

http://rmi.lbi.ro/rmi_2020/

• +60

By lior5654, history, 21 month(s) ago,

Opening this thread to discuss XII International Autumn Tournament in Informatics (IATI) Problems.

Hope everybody had a great competition!

https://iati-shu.org/en/home/

• +18

By lior5654, history, 22 months ago,

As round 696 is getting closer, our chad Um_nik finally achieved 6969 followers on codeforces! Congratulations!

• +372

By lior5654, history, 22 months ago,

I think it would be cool if a user would be able to make a paragraph about themselves and make it appear on their codeforces profile.

Was this feature suggested before? I see no issue implementing it within a few days and it would be cool for it to be added.

• +87

By lior5654, history, 2 years ago,

Opening this thread for discussion of APIO 2020.

Not really sure whether the problems themselves can be discussed yet (open contest) but the competition has officially ended :)

Edit 1: APIO 2020 Scientific Committee note: "As you mentioned, please refrain from publicly discuss APIO 2020 problems as there will still be an APIO 2020 open contest (public). We can publicly discuss the problems once the open contest has finished."

Feel free to discuss results, we'll only be able to discuss problem content after the open contest ends.

Edit 2: Should be good now :)