Algorithms Thread Episode 8: Tree Basics

Episode 8 of Algorithms Thread comes out in <90 minutes! This one is a bit more beginner-friendly and covers the following ideas:

• Graph/Tree Diameters
• Binary Lifting
• Tree Flattening with Euler tours

Also, to make sure you have actually learned that stuff, I made a custom Gym set on CodeForces that will last two weeks that hopefully is really good practice for making sure you have learned this stuff. Here is a link to the gym set; it will be available 45 minutes after the video comes out so that people have time to watch the video before starting the set, if they are interested in penalty points. All of the problems in the gym are original to this set (in their flavortext at least, some are simple enough that I'm sure they have appeared in other contests before).

The new gym integration was heavily inspired by Errichto's Matrix Expo set format. Let me know whether it's helpful. I think it might be, but also it's a pretty big time commitment to make it, so whether I keep doing them depends on how helpful they are to people.

If you have any questions or suggestions, feel free to leave them below. I hope you enjoy the problem statements, and I'll leave you all with this:

Solutions

Update:

• Video solutions to all problems

• Github containing solutions to all problems and data generators

Hi!

On 16.08.2020 17:35 (Московское время) we will host Codeforces Global Round 10.

This is the fourth round of a 2020 series of Codeforces Global Rounds. The rounds are open and rated for everybody.

The prizes for this round are as follows:

• The top 30 participants get a t-shirt.
• 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.

The prizes for the 6-round series in 2020 are as follows:

• In each round, the top 100 participants get points according to this table.
• The final result for each participant is equal to the sum of the points they got in the four rounds where they placed the highest.
• The top 30 participants over all series get sweatshirts and place certificates.

Thanks to XTX for supporting the global rounds initiative in 2020!

The problems in this round were prepared by KLPP, zscoder, qlf9, golions, MagentaCobra, and me. We would like to give a huge thanks to the following people:

• antontrygubO_o for rejecting most of our problems coordinating our round.
• gamegame, dorijanlendvaj, thenymphsofdelphi, aryanc403, Ari, alireza_kaviani, KonaeAkira, Mn619, AmShZ, smax, galen_colin, cuber_coder, gabrielwu, nvmdava, 300iq, Monogon, Mohamed.Sobhy, maroonrk, 244mhq, pajenegod, Devil, Osama_Alkhodairy, Mohammad_Yasser, taran_1407, Um_nik, physics0523, hugopm, mohammedehab2002, gonP, tfg, Joker_11, kshitij_sodani, JAPL, duckmoon99, Blastman, coderz189, and Antebe for testing and providing invaluable feedback.
• MikeMirzayanov for the Codeforces and Polygon platforms.
• All of you for participating in our contest!

We had a lot of testers as the problemset of the round changed significantly throughout testing! As a result of the huge amount of feedback, we think that we've managed to make the round really high quality and hope that you'll enjoy it :)

You will be given 3 hours to solve 9 problems. The score distribution will be announced at some point in time before the contest starts. Good luck!

UPD: Score distribution:

500 — 750 — 1000 — 1500 — 2000 — 2500 — 3000 — 3500 — 4000

UPD: Editorial

UPD: System tests have finished. We hope you liked the problems! We apologize for the weak pretests on A and B — that was a major oversight on our part. Hopefully you were still able to enjoy the contest regardless :)

Congratulations to the winners!

1. boboniu
2. maroonrk
3. ecnerwala
4. tourist
5. Petr
6. ksun48
7. tmwilliamlin168
8. sunset
9. whzzt
10. hos.lyric

Hello Codeforces!

On Aug/14/2020 17:35 (Moscow time) Educational Codeforces Round 93 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

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 Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Neon Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Congratulations to the winners:

Congratulations to the best hackers:

85 successful hacks and 696 unsuccessful hacks were made in total!

And finally people who were the first to solve each problem:

UPD: Editorial is out

The Registration for the traditional international students Petrozavodsk Programming Camp continues. The Camp will be held for the 39th time, the dates are August, 21 — August, 27 2020.

THE CAMP WILL BE HELD ONLINE.

Teams of up to 3 members are invited to join.

The level of contests: Division A / B, i.e. participants of the semi-finals and finals of the ICPC World Championship

Preliminary schedule:
21.08 — Contest 1
22.08 — Contest 2
23.08 — Contest 3
24.08 — Day off
25.08 — Contest 4
26.08 — Contest 5
27.08 — Contest 6

Competitions will be held on the Yandex.Contest platform.
For each contest, an analysis will be broadcast in English.

The registration fee for participation in the Camp for participants from the EAEU countries is 15,000 rubles per team.
The registration fee for participation in the Camp for participants from other countries is $300 per team.

Contact Information

• Contact the organizers: [email protected]
  
• Website: camp.acm.petrsu.ru
  
• Group in VK: vk.com/camp_acm_petrsu
  
• Snark Camp statistics: karelia.snarknews.info
  

The Camp are supported by:

Yandex, general sponsor
AIM Tech, sponsor
Botan Investments, sponsor
Moscow Workshops ICPC, partner
ICPC NERC HQ, partner

Hello Codeforces! We (dqa2021, Xiejiadong, Retired_xryjr233, Z18 and me) are excited to invite you to take part in Codeforces Round 664 (Div. 1) and Codeforces Round 664 (Div. 2), which will happen on 12.08.2020 17:35 (Московское время).

Huge thanks to:

• 300iq for coordinating our round.
• triple__a for answering my questions about contest preparation.
• duyiblue, jhdonghj112, jqdai0815 for providing and discussing ideas.
• gamegame, KonaeAkira, gtrhetr, jqdai0815, mayaohua2003, crasysky, aryanc403, RainAir, dqa2021, dingwanren, world_top_1_AD_uzi, Juanzhang, _Aaryan_, GavinZheng, taran_1407, yijan, whuang369, jhdonghj112 for testing and providing feedback.
• MikeMirzayanov for Codeforces and Polygon platforms.

There will be 5 problems in Div.1 round and 6 problems in Div.2 round. You'll be given 2 hours to solve them.

The story of this round is about that man. Instead of displaying his name, I prefer telling one of his legends (or joke):

"I have a 'friend', who makes lots of money every day, earning a billion in the blink of eyes. With a wave of his hand, OIers all over the world will follow him."

You can post your guesses in the comments.

UPD: Score Distribution:

• Div.1: 500 — 1000 — 1500 — 1750 — 2500
• Div.2: 500 — 750 — 1000 — 1250 — 1750 — 2250

Good luck!

UPD: Congratulations to the winners!

Div.1:

1. Benq
2. ecnerwala
3. nick452
4. 244mhq
5. neal

Div.2:

1. C.S.T.T
2. MyLoveKUN
3. Rchen3
4. Rainbow_qwq
5. evilbuggy

Henlo Codeforces! ^_^

I invite you to participate in Codeforces Round #663 (Div.2) taking place on Aug/09/2020 17:35 (Moscow time). The round is rated for users rated less than 2100, while other users can participate non-competitively.

The round features five problems, and you have 2 hours to solve them. There may, or may not, be an interactive problem; regardless, you should know how to deal with them.

I would, now, like to thank–

• antontrygubO_o for coordination and manipulation.
• AwakeAnay, AsleepAdhyyan, and RestingRajarshi for always listening to my problem ideas.
• BRCode for making 3b1b-style video editorials of the problems!
• Aggu_01000101, Devil, Dragnoid99, far_from_NOOB, IaMaNanBord, Osama_Alkhodairy, prabowo, taran_1407, Tarrus, Tathagat_shah, TheOneYouWant, Utkarsh.25dec, Vivek1998299, _Aaryan_, aryanc403, dorijanlendvaj, coderz189, hugopm, knightron00, kshitij_sodani, ltc.groverkss, nishkarsh, Roham, shash42, socho, talibmohd, and thenymphsofdelphi for testing!
• MikeMirzayanov for Codeforces and Polygon platforms– Polygon is truly remarkable!

Please do not mind the long list of testers (I had to write code to tag everyone here) since the problem set changed significantly after the first round of testing.

We will announce the scoring distribution shortly. The scoring distribution is 500–750–1250–2000–2750.

Good luck, and stay safe!

UPD: Editorial

Here are video editorials by BRCode:

• Problem A and B
• Problem C
• Problem D
• Problem E

UPD2: Finally, congratulations to the winners!

Div. 1:

1. tmwilliamlin168
2. risujiroh
3. fanache99
4. LayCurse
5. Sugar_fan

Div. 2:

1. 00000010100001100111
2. PouyaNavid
3. 420iq
4. I_want_2400
5. silxi

Hello, Codeforces!

74TrAkToR and I are glad to invite you to our Codeforces Round 662 (Div. 2), which will be held at Aug/07/2020 17:35 (Moscow time). The round will be rated for all the participants with rating strictly less than 2100.

The problems were created and prepared by AlFlen and 74TrAkToR. We would also like to thank everyone who helped us a lot with round preparation.

• Our coordinator 300iq for patience and help with ideas.
• Our dear friend lapochka_queen for the idea of one of the tasks.
• Our testers isaf27, Astralpirate, sava-cska, Siberian, voventa, romanasa, antontrygubO_o, DIvanCode, _overrated_, Mlxa, plagues, Prokhor08 for high-quality testing and valuable advices.
• And our team testers: ainta (ksun48, 300iq) и акацуки (p1rattttt, kostyamyasso2002) for proper testing.
• MikeMirzayanov for amazing Codeforces and Polygon platforms.

On the round you will be asked to help main ponies from My Little Pony animated series (Fluttershy, Applejack, Twilight Sparkle, Pinkie Pie, Rarity, Rainbow Dash) and to solve 5 problems, one of which has two subtasks. You will have 2 hours to solve them.

Score distribution will be announced shortly before the round.

UPD: Score distribution: 500 — 1000 — 1500 — 1750 — (1500 + 1500)

UPD2: Editorial

We wish you good luck and high rating!

Hello! Codeforces Round 661 (Div. 3) will start at Aug/05/2020 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

• take part in at least two rated rounds (and solve at least one problem in each of them),
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of my work. Thanks to my good friends Daria nooinenoojno Stepanova, Mikhail awoo Piklyaev, Maksim Neon Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round. Also thanks to Artem Rox Plotkin and Dmitrii _overrated_ Umnov for the discussion of ideas and testing the round!

Good luck!

UPD: Huge thanks to Ivan Gassa Kazmenko for testing the round and fixing some issues with statements and the round in general!

UPD2: Editorial is published!