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2158A — Suspension

2158B — Split

2158C — Annoying Game

2158D — Palindrome Flipping

2158E — Sink

2158F2 — Distinct GCDs

Only text editorial available for this problem.

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2170A — Maximum Neighborhood

2170B — Addition on a Segment

2170C — Quotient and Remainder

2170D — Almost Roman (quick walkthrough)

2170E — Binary Strings and Blocks

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2157A — Dungeon Equilibrium

2157B — Expansion Plan 2

2157C — Meximum Array 2

2157D — Billion Players Game

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2171A — Shizuku Hoshikawa and Farm Legs

2171B — Yuu Koito and Minimum Absolute Sum

2171C1 — Renako Amaori and XOR Game (easy version)

2171C2 — Renako Amaori and XOR Game (hard version)

2171D — Rae Taylor and Trees (easy version)

2171F — Rae Taylor and Trees (hard version)

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2164A — Sequence Game

2164‌‌‌‌B — Even Modulo Pair

2164C — Dungeon

2164D — Copy String

Also, the free bootcamp for 1200 -> 1600 rating starts from today!

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2107A — LRC and VIP

2107B — Apples in Boxes

2107C — Maximum Subarray Sum

2107D — Apple Tree Traversing

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2108A — Permutation Warm-Up

2108B — SUMdamental Decomposition

2108C — Neo's Escape

2108D — Needle in a Numstack

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2094A — Trippi Troppi

2094B — Bobritto Bandito

2094C — Brr Brr Patapim

2094D — Tung Tung Sahur

2094E — Boneca Ambalabu

2094F — Trulimero Trulicina

2094G — Chimpanzini Bananini

2094H — La Vaca Saturno Saturnita

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2093A — Ideal Generator

2093B — Expensive Number

2093C — Simple Repetition

2093D — Skibidi Table

2093E — Min Max MEX

2093F — Hackers and Neural Networks

2093G — Shorten the Array

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2085A — Serval and String Theory

2085B — Serval and Final Mex

2085C — Serval and The Formula

2085D — Serval and Kaitenzushi Buffet

2085E — Serval and Modulo

Note: This is what I did. This is what I always tell my students. This is what I’m sharing because I believe it is very helpful for people here.

1. Are the tips I’m sharing the only way to reach Expert? No.

2. Is it the best way to reach Expert? Not necessarily, but I think so.

3. Can I leave hateful comments below? Do something more valuable lol. Write another blog explaining how you did it, or even a comment works.

4. Are you actually helping or just promoting your video? First, regardless of any benefit to me, this is valuable information for people. Second, I genuinely don’t need the promotion. 32,000 people have already watched it in a week, and this blog might add like 1,000 more, doesn’t make any noticeable difference for me.

Two types of advice

I believe advice generally falls into two types. Some advice is not controversial, so everyone agrees on it. By definition, it is ‘useless’ because it doesn’t add new knowledge. For example, telling someone to “practice” or “train hard” isn’t helpful since everyone already knows and agrees on that. It can serve as a reminder or motivation, but it doesn’t add knowledge.

On the other hand, some advice isn’t something you hear everywhere. Not everyone may agree with it. So, what I did here was share those kinds of opinions. And of course, anyone is free to agree or disagree in the comments. This is an open discussion.

Is there really a correct and incorrect way to learn competitive programming? Definitely. I’ve seen many people do it incorrectly. At the end of this blog, I’ll share experiences I’ve had with my previous students in different schools.

I’ll attach the video I've made at the end, but I’ll also summarize some of the key points for those who prefer reading.

Why am I so confident about this style? I've did it myself. As you can see in my rating graph, I went from newbie to expert in two months (March–May 2015), and after three months, I ranked 4th in a div 2 contest (still my best CP day ever). So, I confidently say that it works.

Okay, so what is it?

For example, I tell people to solve problems around their own rating. If your rating is 1200, solve many 1200-rated problems. It’s not a waste of time. Don’t jump to harder problems too soon. Many people do this, but they end up spending too much time thinking and not enough time implementing. Therefore, they become good at problem-solving but suck at implementation.

Also, don’t think too much. If you’re stuck, feel free to look at hints or solutions, that’s fine! You’re a beginner and you are building intuition. (Some people strongly disagree and think you should never check a solution, even if a gun is pointed at your head. I completely disagree, remember, you have just started.) I recommend choosing problem that on average you spend about 30 minutes per problem. (average! so you might solve some in 10 mins, and some in more than an hour) Some say, “Think on a hard problem for 5 hours and enjoy!” No. trust me, Accepting 10 different problems helps more at this stage. I also recommend reviewing other people’s code. Early on, this helps you write more efficient solutions.

And seriously, don’t start by learning algorithms. Just solve problems. Learning complex algorithms too early is mostly a waste of time. To reach expert, you only need to solve problems with tags like implementation, brute force, math, greedy, and DP. (DP is the only one that requires serious practice.)

Here you can see the full video:

This is a recurrent pattern, happened over and over again in my teaching experience. I start teaching in a school, and I see that after a year the students are still newbie/pupil at most. I work with them and in several months they get to expert. Why? The previous teacher was teaching them AVL tree (or similar things), solving very hard problems, etc. If you ask the students to explain some complex algorithm in a paragraph, they can do that. But if you give them a simple problem to accept, they can't.

Trust me, this is not the correct way to learn CP. And, yes, there is a correct way and an incorrect way.

Feel free to agree/disagree in the comments. It’s welcome and helps everyone to know opposite point of views.

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2070A — FizzBuzz Remixed

2070B — Robot Program

2070C — Limited Repainting

2070D — Tree Jumps

2070E — Game with Binary String

Hi Codeforces,

Our last topic stream was on Combinatorics. We talked about exponentiation, combination (+ why does it have that formula), what is a combinatorial proof (the most interesting part), and proved Pascal's equality and a bunch of other problems using that. Then, we talked about how to calculate combination with O(N) preprocess time and O(1) query time, and finally, solved 559C — Gerald and Giant Chess.

Exponentiation ($$$a^b$$$) in $$$O(lgb)$$$

Combination $$$n\choose k$$$ and why is it $$$\frac{n!}{k!(n-k)!}$$$

Algebraic proofs suck, what is a Combinatorial Proof

(+ Proving a bunch of equalities like Pascal's, using combinatorial proof)

ChatGPT trolling me

Calculating combination in code

simplest: $$$O(n^2+q)$$$

better: $$$O(n+qlgn)$$$

even better: $$$O(nlgn+q)$$$

not that much different: $$$O(n + q)$$$

Solving 559C — Gerald and Giant Chess

(Combination + DP)

This is the link to choose the topic for the next topic stream:

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2065A — Skibidus and Amog'u

2065B — Skibidus and Ohio

2065C — Skibidus and Fanum Tax

2065D — Skibidus and Sigma

2065E — Skibidus and Rizz

2065F — Skibidus and Slay

2065G — Skibidus and Capping

2065H — Bro Thinks He's Him

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During today’s contest solution discussion, we tested ChatGPT o3-mini-high on our contest problems. And get this: it solved problems C and D! It claimed that it did it without internet access.

We asked about its capability, and it said that if it made a Codeforces account, its rating would be 3500–4000! I don’t know if AI has learned to solve very hard problems or if it’s getting overconfident.

All I can say is, things are getting pretty SCARY. I have no idea what’s going to happen with CP contests and competitive programmers soon.

2059A — Milya and Two Arrays

2059B — Cost of the Array

2059C — Customer Service

GPT o3-mini-high Solved C

2059D — Graph and Graph

GPT o3-mini-high Solved D

Asking ChatGPT if it is Cheating (Has Internet Access)

o3 claiming that it has rating 3500 on Codeforces!

Trying to Solve E2 with o2 — Not Successful

P.S. Seems like rank 4000+ should be called GPT soon.

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2060A — Fibonacciness

2060B — Farmer John's Card Game

2060C — Game of Mathletes

2060D — Subtract Min Sort

2060E — Graph Composition

2060F — Multiplicative Arrays

2060G — Bugged Sort

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2050A — Line Breaks

2050B — Transfusion

2050C — Uninteresting Number

2050D — Digital string maximization

2050E — Three Strings

2050F — Maximum modulo equality

2050G — Tree Destruction

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2042A — Greedy Monocarp

2042B — Game with Colored Marbles

2042C — Competitive Fishing

2042D — Recommendations

2042E — Vertex Pairs (not complete)

2042F — Two Subarrays

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2034A — King Keykhosrow's Mystery

2034B — Rakhsh's Revival

2034C — Trapped in the Witch's Labyrinth

2034D — Darius' Wisdom

2034E — Permutations Harmony

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2037A — Twice

2037B — Intercepted Inputs

2037C — Superultra's Favorite Permutation

2037D — Sharky Surfing

2037E — Kachina's Favorite Binary String

2037F — Ardent Flames

2037G — Natlan Exploring

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2028A — Alice's Adventures in "Chess"

2028B — Alice's Adventures in Permuting

2028C — Alice's Adventures in Cutting Cake

2028D — Alice's Adventures in Cards

2028E — Alice's Adventures in the Rabbit Hole

2028F — Alice's Adventures in Addition

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2029A — Set

2029B — Replacement

2029C — New Rating

2029D — Cool Graph

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2036A — Quintomania

2036B — Startup

2036C — Anya and 1100

2036D — I Love 1543

2036E — Reverse the Rivers

2036F — XORificator 3000

2036G — Library of Magic

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2032A — Circuit

2032B — Medians

2032C — Trinity

2032D — Genokraken

2032E — Balanced

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2026A — Perpendicular Segments

2026B — Black Cells

2026C — Action Figures

2026D — Sums of Segments

2026E — Best Subsequence