Recently, I've had a wish to sit down with great competitive programmers that I find interesting and pick their brains about different topics. For the first iteration, I talked to my good friend Pajaraja, as I'm sure he's unknown to many of you, but he's well known in Serbia as one of the best — both problemsetters and competitive programmers (and especially competitive mathematicians).

Some of his accomplishments:

• 2x Gold medalist at the International Mathematical Olympiad (as well as a Silver medalist)
• Silver medalist at the International Olympiad in Informatics
• 3x ICPC World Finals participant (including a recent 15th place in Luxor)
• IOI Problemsetter (IOI 2021 — Fountain Parks)
• Google HashCode finalist
• many more!

We discuss differences between competitive math and competitive programming, problemsetting, his motivation, pressure and success among other things. Link in case the embed doesn't work.

As a special treat, we decided to prepare a problemset from some of the best tasks Paja set over the years. For most of these, this is the first time they have been translated to english and shared publicly. They have been selected by Paja and me to have a variety of difficulties, from div2C to div1E/F — we believe everyone (from newbies to legendary grandmasters) will find something interesting to do.

We prepared two versions (with same tasks) — one is sorted (according to our thoughts), with subtasks (for individual practice), the other is shuffled, with ICPC scoring — this version is great for people practicing for team competitions. You can find both versions in this group. If enough people do it and like it, we can do a follow up problem analysis :)

Even if you don't have the time to watch our discussion, I highly recommend you check out the problemset!

Our team qualified for ICPC World Finals 2022 on November 21st, 2021 at SEERC. We will participate at the World Finals this Thursday, April 18th, 2024 in Luxor. We've been thinking how much time has passed since then and about all the things that occurred since. Here are a few of my notes:

• 879 days have passed, or 2 years 4 months and 28 days
• Got engaged and married
• My rating dropped by 116 points
• I gained 35kg
• My teammates moved into two different countries to study
• Novak Đoković won 4 Grand Slams, 3 Masters and 2 ATP Finals
• Two major international wars were (re)started
• Messi became World Champion
• India became most populous country
• Queen Elizabeth died
• Everybody started caring about AI again

I'm sure many of you have the same experience as us, leave any cool facts in the comments :)

Best of luck to everyone, especially those not directly competing with us!

P.S. Don't take this as a dig at ICPC — while I dislike this long pause, some of the stuff has been really bad luck on their side. I'm thankful to finally have the opportunity to participate at this grand event. One thing they could fix is not knowing whether we qualified until last August...

Since the second round of this season of COCI (Croatian Open Competition in Informatics) was held last saturday, here is the continuation in the series of COCI appreciation posts (I will post one after each COCI round, so six in total!). For my coaching, I went through all COCI contests since 2006 to gather tasks and selected some of my favorite ones.

Notes

• Check out the first post for notes about the process and the problemset.
• There are 9 tasks in this problemset since I accidentally forgot to put one task (Zoo) last time, so it's included now.

Tasks

A: 2016/2017, Contest 5: Ronald

Author: Adrian Satja Kurdija (satja)

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Difficulty range: 1300-1500

B: 2011/2012, Contest 2: Kompići

Author: Adrian Satja Kurdija (satja)

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Difficulty range: 1300-1500

C: 2013/2014, Contest 2: Putnik

Author: Adrian Satja Kurdija (satja)

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Difficulty range: 1500-1700

D: 2020/2021, Contest 5: Planine

Author: Daniel Paleka and Paula Vidas (dpaleka,paula)

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Difficulty range: 1700-1900

E: 2013/2014, Contest 2: Linije

Author: Domagoj Ćevid

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Difficulty range: 1900-2100

F: 2019/2020, Contest 1: Trobojnica

Author: Daniel Paleka (dpaleka)

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Difficulty range: 2100-2300

G: 2014/2015, Contest 4: Stanovi

Author: Mislav Balunović (mislav)

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Difficulty range: 2100-2300

H: 2016/2017, Contest 6: Sirni

Author: Domagoj Bradač (DBradac)

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Difficulty range: 2200-2400 (my guess :) )

Bonus task (since I forgot it last time):

2019/2020, Contest 1: Zoo

Author: Ivan Paljak (ipaljak)

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Difficulty range: 2200-2400

I hope you find these tasks useful!

Since a new season of COCI (Croatian Open Competition in Informatics) started last saturday, here is the first in a series of COCI appreciation posts (I will post one after each COCI round, so six in total!). For my coaching, I went through all COCI contests since 2006 to gather tasks and selected some of my favorite ones.

Notes

• I skipped a lot of the really hard tasks because they weren't useful for my students. I kept a list of these tasks, if someone wants it, they can PM me. Also, I only considered COCI, not COI. This current season is not being considered.
• There are many great tasks outside of the 48 shown (throughout the series), so it's worth it to go through other tasks as well. I have selected just my favorite ones.
• These tasks are maybe not always the most innovative, I have selected the tasks that are most useful for coaching and learning new ideas, so for people who already know the idea, the difficulty might be much lower. Still, there are some really interesting tasks.
• You can do these tasks either individually or as a virtual contest. There is a lot of them, so I wouldn't recommend less than 3h.
• The notes are meant for coaches, to know if they can use that task for a lecture that they are preparing. They can also serve as hints for competitors (but sometimes they just give out the solution). Both the tags and the notes should be considered spoilers.
• The given difficulty ranges might be completely wrong as I might be out of touch with the easier grades, they are just given as a loose guide.
• Most solutions can be found on the official site by looking up the appropriate season and contest. If not, I'll try to provide the best resource I can.
• Let me know if you know some missing info (author/author cf handle).

Tasks

A: 2015/2016, Contest 4: Deathstar

Author: Dominik Gleich (Dgleich)

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Difficulty range: 1100-1300

B: 2013/2014, Contest 1: Organizator

Author: Domagoj Ćevid

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Difficulty range: 1300-1500

C: 2017/2018, Contest 5: Birokracija

Author: Ivan Zufić (falanga)

Solution (thanks bmerry)

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Difficulty range: 1400-1600

D: 2009/2010, Contest 7: Svemir

Author: Goran Žužić (Zuza)

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Difficulty range: 1600-1800

E: 2014/2015, Contest 6: Kratki

Author: Dominik Gleich (Dgleich)

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Difficulty range: 1600-1800

F: 2019/2020, Contest 6: Index

Author: Marin Kišić (mkisic)

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Difficulty range: 2100-2300

H: 2016/2017, Contest 1: Vještica

Author: Dominik Gleich (Dgleich)

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Difficulty range: 2200-2400

I hope you find these tasks useful!