We invite you to participate in CodeChef’s November Lunchtime, this Saturday, 28th November, from 7:30 pm to 10:30 pm IST onwards 3 hours, 5 problems.

We will also be hosting two **live problem discussion sessions** on Sunday (29th) from 5pm to 6:30pm IST (first 4 problems) and on Monday (30th) from 5pm to 6:30pm IST (last 3 problems), where our panelist, rajarshi_basu will discuss the Lunchtime problems. Find more details here.

Also, if you have some original and engaging problem ideas, and you’re interested in them being used in CodeChef's contests, you can share them here.

Joining me on the problem setting panel are:

- Setter & Admin: Ildar 300iq Gainullin
- Tester: Nikolay Nebuchadnezzar Budin
- Editorialist: Alei Morphy Reyes
- Video Editorialists: Chirayu Jain, Prachi agarwal19 Agarwal, Darshan darshancool25 Lokhande, Yashodhan Molotov Agnihotri, Bharat Singlai, Aryan Aggu_01000101 Agarwala, Radoslav radoslav11 Dimitrov
- Statement Verifier: Jakub Xellos Safin
- Mandarin Translator: Qingchuan UoA_ZQC Zhang
- Vietnamese Translator: Team VNOI
- Russian Translator: Fedor Fedosik Korobeinikov
- Bengali Translator: Mohammad solaimanope Solaiman
- Hindi Translator: Akash Shrivastava

**Prizes:** Top 10 Indian and top 10 Global school students from ranklist will receive certificates and CodeChef laddus, with which they can claim cool CodeChef goodies. Know more here. The video editorials of the problems will be available on our YouTube channel as soon as the contest ends. Subscribe to get notifications about our new editorials.

Good luck and have fun!

300iq orz

sir please make sure to add some dp problems.

Highly Unbalanced contest! Were you really employed to make codechef better than it was before??

Why problem difficulty gradient is so bad at codechef ?

Finally reached Div1!

JK. I enjoyed the problems, thank you.

Spent two and a half hours figuring out that a fake eulerian cycle edge can be between vertices in the same component. :(

The constraint of Chinese version of Circle Coloring is different to English version. And the translator say it's a mistake made by Codechef team because their final constraint it different to the constraint in repository...

There was an announcement during the contest about these constraints.

WoW, I don't receive any notice about it. Now I see when I participate Codechef Contest, I should read the announcement frequently.

CodeChef supports CF like notifications. One has to enable them. Sometimes a popup comes asking if you want to enable them or not. If you use chrome you can search how to turn on notifications for a website.

That would be nice if we can receive it, but are you sure?

I confirmed that I allow notification for CodeChef and I've participated in many contests but I've never received it.

https://support.google.com/chrome/answer/3220216?co=GENIE.Platform%3DDesktop&hl=en

Every now and then I receive notifications of unacademy CPAL on CodeChef.

Hence I assumed there are contest notifications as well. Maybe setters/admins are not aware of this?

/cc 300iq .

Is there a linear solution for B? I spent the entire contest optimizing my N log N on B... and I was only able to get it down to like 1.2s with bitset cheese.

The answer is sum(d(i)*d(i+1)/2-1), where d(n) is the number of divisors of n.

I see. Couldn't figure that part out :(

In Fractions, we are supposed to count the number of divisors of $$$i \cdot(i+1)$$$ which are $$$ \leq n-i $$$ and add them into the answer for each $$$ i, 1 \leq i \leq n $$$. Why, the answer is always $$$ = \sum_ {i=1}^{n} ( \frac{nod(i) \cdot nod(i+1)} {2} - 1 ) $$$, where $$$nod(i)$$$ = number of divisors of $$$i$$$ ?

Gasoline — fine but very binary-scored, I can't imagine an easy-ish solution for partial points

Fractions — interesting problem but tight limits. Why only ten points for $$$N \leq 10^5$$$? And did you want $$$O(N)$$$? My $$$O(N \cdot \log N)$$$ sieve was too slow. I ended up binary searching tests (which were just $$$10^6$$$ and $$$10^6-1$$$) and hardcoding solutions.

Rooks — standard graph problem

Coloring — weak tests, many people got 80 points with a solution that doesn't work for small $$$N$$$. And why no subtask for a small sum over $$$|A_i|$$$?

Eating — fine but why limits $$$N \leq 20$$$, $$$\sum N \leq 200\,000$$$ for brute force? $$$O(2^N)$$$ will get TLE.

This is for sure not a good contest for people who prepare for IOI. Please spend some time making subtasks. You can even use fewer problems then.

In Fractions, The editorial describes $$$O(N \cdot \log N)$$$ solution too. I checked that tester's solution works in 0.5s because it does something non-trivial to reconstruct divisors instead of just storing them. So why set TL to 1s instead of 2-3s? Come on, at least give us 90 points for your complexity with a worse constant factor.

In Coloring, the editorial actually describes a solution that consists of parts for small and big $$$N$$$. So you knew that big $$$N$$$ can be solved separately and you didn't make a subtask for that and instead made us guess (which was easy by looking at standings) that you didn't put tests with small $$$N$$$ in subtask 2.

On the bright side, I very much like the solution for Coloring!

Regarding Fractions, we can use some memory optimizations (implementing a vector with reserved memory without freeing memory at exit) to fit it in around 0.97 seconds (it barely fits anyway), and I agree that a very slightly worse constant factor doesn't warrant a 90 point penality.

For Fractions, my complexity is $$$\mathcal O(N \log \log N)$$$ I think, you can see my submission here. It runs in 0.12 seconds, but yeah I agree that there was no reason to make the bounds so high and the time limit so low.

Looking at the number of submissions, seems like the contest was made only for GM+ level people

Video editorials for 3 problems have been uploaded here. The other 4 videos will be uploaded over the next day or two.

bad contests no dp problems

metaironic?

no