Self-deception: maybe why you're still grey after practicing every day I generally don't like to give much advice on "how to practice", but recently I have been thinking a lot about this and I realized that there is something that I believe affects a lot of people (though it is admittedly hard to tell) that barely gets talked about: self-deception. Self-deception is a devious thing. It can be very hard to detect, yet can be fatal to improvement. This blog is mainly directed to greys, greens and cyans that can't get out of these categories. Most examples are given with that in mind. That being said, I believe others have something to think about as well. This is my first blog on such "psychological issues" and I don't know if I'll ever make another one. It's certainly a difficult topic to write about because for most of the blog, the message is "your practice is not as good as you think it is" or even "you aren't as good as you think you are" which can of course be a hard thing to hear. Thus, I have to somehow be very gentle while getting the poin...

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C++ quirks: variable shadowing C++ is a very permissive language. It'll let you write basically anything on the assumption that you know what you're doing... or let you shoot yourself in the foot if you don't. For example: ```cpp class C { int a; public: C(int a) : a{a} {} }; ``` It's a fairly common pattern in simple class design that's perfectly fine until it's not. The class member and constructor argument (c.arg.) have the same name. This is known as shadowing and compilers will warn you about it, though you need to use a flag `-Wshadow` that isn't included among `-Wall` or `-Wextra`. Another example of shadowing is ```cpp for(int i = 0; i < t; i++) for(int i = 0; i < n; i++) ``` which is fine if the first `i` isn't used for anything except making a loop (such as over test cases) but otherwise it's a solid source of bugs, especially if you "fix" it into `for(int j = 0; j < n; i++)`. ### How does shadowing work? First off, when can one variable shadow another? The standa...

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This one weird trick will speed up your I/O 5x Just do this!! ```cpp #include <iostream> int main() { std::ios_base::sync_with_stdio(false); std::cin.tie(nullptr); // Your code here int n; std::cin >> n; long long sum = 0; for (int i = 0; i < n; i++) { int x; std::cin >> x; sum += x; } std::cout << sum << std::endl; } ``` This will speed up your code so much you won't have to care about spending time in I/O anymore!! Give likez plz :3 [cut] <!-- --> --- Yeah, no. That's not how it works. We're still using `<iostream>`, and that means we have to rely on virtual inheritance and all the deoptimizations that follow from that. Even if you use a random "here's-my-fast-io-library" snippet from Codeforces, you still depend on block-buffered output. (Also, none of those libraries mesh well with floats.) In many cases, more CPU time is spent encoding/decoding numbers than interacting with the file system. Can we do better? ...Yeah, stupid que...

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Первоапрельский контест 2019: разбор задач Этот контест оказался самым сложным из всех Первоапрельских контестов до сегодняшнего дня &mdash; победитель решил всего 5 задач из 7! ##[problem:1145A] Самая простая задача контеста, с условием и без подводных камней &mdash; мне просто показалась забавной идея сортировки массива методом уничтожения неугодных элементов :-) [cut] Поскольку в массиве не более 16 элементов, достаточно в точности реализовать алгоритм, описанный в условии: проверить, отсортирован ли массив, если да, вернуть его длину, иначе вернуть максимум из длин результатов для его левой и правой половин. ##[problem:1145B] Ответ YES или NO подсказывает, что нужно определить, является ли данное число числом kanban; но сначала хорошо бы выяснить, что такое числа kanban. Этой последовательности нет в OEIS, но там есть несколько похожих, например, [urban numbers](https://oeis.org/A072957) и [turban numbers](http://oeis.org/A072956). Эти последовательности состоят из чисел, которые "банят" определенные буквы, т...

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Week 2-24 It's been 6 months since I've registered for codeforces and I've decided to continue updating my progress! Big thanks to [user:SecondThread,2022-09-09] for the support shown in my first blog. #### Overall Changes 154 problems solved(+~134 problems), 1154 rating(from unrated), Switched from python to Java (learning AP Computer Science A)! I've also done ~80 problems on usaco guide and usaco and participated in 40 cf contests(rated and unrated), I try to participate in every contest I could attend. Generally the style of codeforces problems appealed more to me than other competitive programing sites. Although I haven't gotten much better at solving harder problems, I've become much better at implementing algorithms. Still have a long way to go from here. Week 2-6 ------------------ Best solve: [submission:151053765] First time solving C and D in a diff 2 contest(upsolving)(and also haven't been able to do this since). Got very lucky with figuring out the quirks of the pr...

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BitShift 2026 Editorial BitShift 2026 Editorial ================= Hello everyone, here is the editorial for [BitShift 2026 Contest](https://codeforces.me/blog/entry/150003) conducted by [Programming Club, IITM](https://progclubiitm.com/). Thank you for participating in the contest. Hope you had fun solving the problems. All of the problems were original and we would be happy to hear your feedback in the comments and in the mini-survey below. <spoiler summary ="How did you like the contest overall"> - Excellent: - Good: - Mediocre: - Bad: - Terrible: </spoiler> <spoiler summary="Which problems did you like?"> - [A. Complete Subarrays](https://codeforces.me/gym/664897/problem/A) - [B. Minesweeper](https://codeforces.me/gym/664897/problem/B) - [C. Div. 2,3,4,5 Problem](https://codeforces.me/gym/664897/problem/C) - [D. Supercharging Nodes](https://codeforces.me/gym/664...

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Unnecessary Details: 1B — Spreadsheet [problem:1B] was extremely challenging for me; it took me over 2 hours to get AC. I didn't find the editorial helpful, and then when I went to see practice solutions from red/orange people, it made even less sense (they were basically copy/paste with minor variation of the following): <spoiler summary="Solution"> ~~~~~ #include<cstdio> void g(int t){if(t){g((t-1)/26);putchar(65+(t-1)%26);}} int main(){ int n,x,y; char s[64],*p; for(scanf("%d ",&n);n--;){ gets(s); if(sscanf(s,"%*c%d%*c%d",&x,&y)==2){ g(y); printf("%d\n",x); }else{ for(x=0,p=s;*p>64;++p) x=x*26+*p-64; printf("R%sC%d\n",p,x); } } return 0; } ~~~~~ </spoiler> I'll talk about each part of the problem itself and approaches/alternatives, and then explain how the above solution (henceforth "the solution") handles it. I'll go into unnecessary detail since I'm very new to CP/CF and would've wanted this...

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[Tutorial] The sparse set data structure <big><big>Hello, Codeforces!</big></big> I recently came across a data structure that I find quite interesting: the **sparse set**. It functions similarly to a bitset and supports all its operations but has some quirks and useful features of its own. Particularly, unlike almost every other data structure, it does not need to be *initialised* at all before it can be used! Naturally, that also means that its data can be reset in $O(1)$ time, as opposed to the $O(n)$ time expected of regular bitsets. It also has the added benefit that traversing its elements takes time proportional to the number of elements rather than the size of the data structure, making the operation more efficient for sparse bitsets. However, note that it is likely less efficient in terms of its memory usage (still $O(n)$ but ~64× larger constant) and constant time factor. ## Introduction The sparse set consists simply of two integer arrays (which I'll call $d$ and $s$) and a size variable $n$, satisfying th...

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Haskell CF gotchas as a beginner I got into using CodeForces a few weeks ago, using Haskell for the last two competitions. I spent hours debugging three Haskell quirks in the competitions, so I hope this saves time for future Haskellers! 1. Do NOT use default `getContents` or `getLines` in CodeForces. I found that at least one of my solutions per competitions will TLE even if I have the perfect algorithm. The issue was not my algorithm, it was the I/O time. Switching to `getContents` or `getLines` from `Data.ByteString.Char8` fixed these issues. I usually do something like ```hs import qualified Data.ByteString.Char8 as BS main = do n <- fst . fromJust . BS.readInt <$> BS.getLine forM [1..n] $ \_ -> do input <- BS.getLine -- logic ``` 2. STRIP OUT `'\r'` from the end of the output of `Data.ByteString.Char8.getLine`. I spent hours trying to figure out why my one of my string-based algorithms worked on my laptop, but not CF. It turns out that newlines on CF are `\r\n`, while ...

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Flexing Cheater Hi Codeforces community and moderators, I am writing this blog to report a clear case of AI usage and plagiarism by [user:Fihh,2026-10-28] in the recent Div. 2 contest ([standings participant page](https://codeforces.me/contest/2272/standings/participant/249331500#p249331500)). Note: Translated with AI assistance; all ideas and writing are my own. While cheating in the LLM era is unfortunately common, what prompts me to make this public post is the user's continuous self-boasting and arrogant comments/blogs about their performance, while heavily relying on AI for their contest submissions. Below is the detailed analysis of [user:Fihh,2026-10-28]'s submissions. --- ### **Key Evidence & Suspicious Patterns** #### **1. Sudden Inconsistent Coding Style** - Across almost all previous submissions, [user:Fihh,2026-10-28] consistently declares raw global arrays directly using hardcoded bounds like `char a[200005], b[200005];`. - However, in submission [submission:396136...

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CF Quirks: Enhance Your Codeforces Experience #### CF Quirks A Chrom(e|ium) extension for customizing the way users and comments are displayed #### Features 1. **All people are equal:** All users will have the same rank (Human) 2. **Bad comments are good:** Bad comments won't be hidden by default 3. **Blacklist:** Hide comments from certain users (with regex to fight against multiplying alts!!!) #### Suggestions I'm open to new suggestions and feature requests (as long as I can implement them of course!) #### Link [Web Store](https://chrome.google.com/webstore/detail/cf-quirks/hiiclkknicjencpfkgmcjnkckonccnmh)

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GNU G++ bug with global set Me and ~MrOkasha,2019-06-03 found an odd quirk with the GNU G++ compiler, (we tested with G++11, G++14, and G++17, all having the same bug). This code here:<pre><code> #include <bits/stdc++.h> using namespace std; set<double> st; int main() { double x, y; for(int i = 0; i < 3; ++i){ cin >> x >> y; st.insert(y/x); } cout << st.size() << endl; } // input: 5 -2 5 -2 5 -2 </code></pre> should output without any doubt '1', as the set should remove duplicate numbers. The output is 3 for some reason on the GNU compilers. It should be noted that the output is indeed 1 on Clang++17 compiler, Visual C++, and other available versions of GNU G++ compilers found online, for us, it seems the bug only occurs here on codeforces, verified by the Custom Invocation feature. The weirdest thing about this bug is that, for some bizarre reason, if the declaration of the set `set<...

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[Tutorial] : Quirks of Non Existent Map Elements I created a blog that talks about 3 quirks of maps/multisets. 1. Accessing non existent elements in maps keyed on characters may lead to WA. 2. Accessing non existent elements in maps keyed on integers may lead to TLE. 3. Using the fancy count operator in multisets may lead to TLE. There's no new/unique insights, I just documented my personal experience with these quirks, along with the problems link where I first encountered this issue. It might be helpful to beginners in competitive programming. https://cfstep.com/training/tutorials/general-techniques/quirks-of-non-existent-map-elements/

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Surprise! [My surprise in the latest Surprise Language Round #8](http://codeforces.me/blog/entry/45454?#comment-300476) got me thinking. What is a surprise? Everybody is surprised by different things. If someone is new to competitive programming, there is an easy way to surprise him: just let him multiply two numbers. The constraints don't even have to be 10^9, 10^5 is enough:) But surprising more experienced coders is much harder. I like the problems that have some element of surprise or anything non-standard. The problems that somehow force the contestant to dig deeper. Something more than just reading the problem statement and converting it to DP. In the regular rounds most often the surprise comes from the missing knowledge about the particular algorithm. If I didn't know about the algorithm, I can google it after the contest, study it, add to personal library and wait for the next contest with similar problem. How can a problem statement be even more surprising? There is a link betw...

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Surprising Python quirk I have two almost identical submissions getting a substantial difference in execution time: [TLE submission](https://codeforces.me/contest/1716/submission/188265769) uses variable MOD inside the main function: ~~~~~ def main(): MOD = 998244353 # local var ... main() ~~~~~ and the [AC submission](https://codeforces.me/contest/1716/submission/188265639) uses variable MOD outside: ~~~~~ MOD = 998244353 # global var def main(): ... main() ~~~~~ Any idea what causes such a difference in performance?

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Got this question in OA yesterday,can anyone solve this Given an array of size n.You need to select those two different indices such that quirk between them is maximum.Quirk Q(i,j)= 

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