Hello all , I think this contest was much easier with the most difficult problem getting more than 100 successful submissions . I think the long contest should have 2 HARD problems definitely . That keeps up the excitement of long contest .

@vineetpaliwal another thing that could make more interesting should be partial scoring - like Hackerrank and other platforms.

I agree. This month’s contest was way easier than any other codechef challenge I have competed in before. Usually for some of the harder problems it takes 1-2 days for me to solve. This month the hardest problem was SPMATRIX but i’ve faced way more hard problems on codechef before. Though the problems were very nice, this month’s contest just didn’t seem challenging enough and that’s what I actually like about codechef challenge. I think there should have been more HARD problems in the contest. The hard problems make you learn a lot of new algorithms and techniques. I just got bored after solving all of the problems. This month just didn’t cut it for me. I hope we get more challenging problems in the future

I believe the main reason was the absence of algorithms that could be implemented using advanced data structures like rope, suffix tree, lazy propogation etc. The hardest problem relied on a series, that could be easily found on oeis, once you brute force the third element (205) and then everything becomes easy. Yes the time limit tortured us a bit, but almost everyone who got till here, would have handled that. So this was not that difficult. Although @Anton_Lunyov you are one of the most talented programmers of our time (and one of my ideals ), I believe you should have gone with an algorithmic challenge instead of a computational one. And why are you so involved with the sequences on oeis? Even on mathwizard, the problems set by you are series from oeis.

“but almost everyone who got till here, would have handled that”

This hurt me real bad :[. At the point where I only needed to eliminate few mods, I took fancy alternatives

@nims11 yes but fortunately I knew they were the problem, since in the last long challenge the Juno problem was similar, I was getting TLE for a long time, then i reduced some mods, and it was accepted. Some play with mods and my runtime was half of the last time. So that was the one that led to reduction of mods and changing some multiplications into additions. That said, an algorithmic challenge would have been better.

There is already a discussion about that here: http://discuss.codechef.com/questions/11340/why-no-partial-grading-in-contest-problems-discussion

The funny thing that I submitted SPMATRIX as MEDIUM since I understand that the problem for finding just one sequence can’t be HARD according to codechef standards. While HARD problems here should be PERMUTE and TKCONVEX. Probably TKCONVEX could be really HARD but weak test data play a trick on us

Also I was aware of presence of my sequence in OEIS, but I decided that hint there is not enough to solve the problem. Well, what can I say - I was wrong

But definitely there were a lot of groups of coders who solved SPMATRIX together (especially among Indians). So the actual number of AC-users is way smaller. And I think if not that crappy hint on OEIS about “fractional iteration” it could be really HARD. @vineetpaliwal As a topic starter could you please confirm that the problem is not challenging enough without this hint?

P.S.

@aichemzee

Which of my two problems set at MathWizard is OEIS sequence?

@anton_lunyov : I solved all problems within the first 24 hours except for SPMATRIX . So obviously this was the hardest problem for me . PERMUTE , i think is definitely not hard , could be called Medium .

In SPMATRIX , i didn’t come across the OEIS sequence as I was able to figure out a way to count sequences through a recurrence relation so no need to “google” about it . However the recurrence I figured out was taking O(n^2) time for computation . But it helped me verify that my recurrence is correct by solving correctly for n = 10.

After that came the phase of going from recurrence to a closed formula . Struggled initially with TLE even with closed formula , but was able to overcome it in 1 hour with optimizations in implementation .

It took me 2 days after the first 24 hours to do this problem correctly . I would like to point out that my solution i guess is not as big as the editorial for this problem . But my solution is based on simple concept which gives recurrence which can be solved . No advanced concepts of mathematics needed .

The reason I started this topic , was because apart from SPMATRIX all other problems struggle to be called Medium , far so to be called hard .

I tried thinking about SPMATRIX for 3-4 days stubbornly refusing to search on OEIS for the first few values. I did not get *any* idea which lead towards the solution :(. So frustated with that, I calculated the first few values and found the sequence on OEIS and finally solved it by translating the mathematica program given there. I am eagerly waiting for the editorials to understand it.

It might be easier contest for you but the question like DELISH, LEMOUSE, PERMUTE were very interesting problems i solved.

@vineetpaliwal What is the recurrence that you derived? Without using OEIS or advanced maths? I am all ears…

DELISH, LEMOUSE, PERMUTE were nice problems but they were easier than that of codechef standard.

The editorial for SPMATRIX is already posted:

Hi Community. Everybody say that the contest was very easy because It has a lot of AC solutions but It is a long contest and people share ideas to everybody and you can see it in websites like http://www.careercup.com. At the end, sometimes, a lot of person sends AC solutions from others users and you can see it in the solutions list for each problems when the contest has finished.

I am curious as well, could you explain your solution ?

Very similar case in **2014** too. I got confused initially!

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