### 300iq's blog

By 300iq, 2 weeks ago, ,

Hello everyone, this summer at Petrozavodsk was my second ICPC contest, now it will be held as OpenCup round.

XX Open Cup Grand Prix of Kazan takes place on Sunday, September 8, 2019, at 11:00 AM Moscow time

The link to the contest. You need an Open Cup login to participate.

To ensure Codeforces traditions, I will say thanks to the testers Benq, wxhtxdy, whzzt, sunset, TLE, ko_osaga, rushcheyo, jiry_2, gamegame, CauchySheep. Also thanks to izban with help in tasks choosing.

UPD: Now the contest is loaded to the gym!

• +257

 » 13 days ago, # |   +31 The contest will start in less than 30 minutes!~
 » 13 days ago, # |   +74 Thanks, everyone, for participating! Editorial pdf: link
•  » » 12 days ago, # ^ |   +30 Can you give the pdf link of problems, its hard to login.
 » 13 days ago, # |   +41 Hello! I was the tester, and I really liked problem H here. It is easily my best problem of 2019. Please solve it!
•  » » 13 days ago, # ^ |   +69 (^з^)-☆ ♥ ♥ ♥ ♥ ♥ ♥
 » 13 days ago, # |   +38 My D passed a stress test but failed on test 9. Do anyone have any idea for the reason?
•  » » 13 days ago, # ^ | ← Rev. 2 →   +28 10 11 3 1 2 1 3 1 8 1 9 3 4 3 5 4 5 6 7 6 8 7 8 9 10 Answer is 4.
•  » » » 13 days ago, # ^ |   +41 Finally I found a stupid bug and ACed. Thank you for the nice problem set:)
•  » » 12 days ago, # ^ | ← Rev. 2 →   0 On the other hand, we got AC on problem A which failed our stresstests 8) (not sure if it satisfied input constraints but we were treating input just as a chordal graph of treewidth <=3) But we managed to find the but afterwards
•  » » » 12 days ago, # ^ |   0 Maybe your algorithm of recognition is not correct for arbitary chordal graph with small treewidth?
•  » » » » 12 days ago, # ^ |   0 That part was trivial. Just find a vertex whose neighbourhood is a clique and remove it . Moreover, naive way of recognition gives you n^2 time, but you can do it with lexbfs in linear or n log n time (I don't remember), what gives a much better complexity than what was written in editorial, since the latter dp part is linear. But anyway, that linear dp part has constant factor sth like 100000, so yeah xD. Our bug was in processing the Join nodes, but it was very hard to find a test where it fails (it was once per a few thousands in our stresstests), so I am not surprised we managed to pass system tests. We didn't think if it is possible to find a failing tests that is compliant with problem constraints, but this should be possible.
•  » » » » » 12 days ago, # ^ |   0 Well, if with arbitary chordal graph you found it with several thousand iterations, probably Apollonian network will require much more iterations :)So it was hard for me to predict your bug :(Anyway, hope you liked this problem xd
•  » » » » » » 12 days ago, # ^ |   +10 I was the one to read it and state "well, it's rather obvious — just standard connectivity dp over bounded treewidth" and delegated coding it to Marek xD. So well, let's say that coding these is kind of a masochistic pleasure, like geometry which I forced myself to enjoy xD. And we can brag afterwards that we actually coded on a competition something that nobody sane ever attempts to code xD.
•  » » » » » » » 12 days ago, # ^ |   0 Real masochistic pleasure comes from the first solution of J. I spent slightly more than a full day to solve it in that way. Everybody should upsolve J in that way XD
•  » » » » » » » » 11 days ago, # ^ |   +20 For even more 'pleasure', solve it in basically the same way, but with a LinkCutTree instead of HLD (which I just did).
 » 13 days ago, # |   +103 I also love the problems, especially F, H and J. One of the best rounds ;)
•  » » 12 days ago, # ^ |   +20 I really liked F and especially H as well!
 » 12 days ago, # |   +18 In K, isn't there $K^2$ possible candidates? Doing Inclusion-Exclusion over that many elements is too much right?
•  » » 12 days ago, # ^ |   0 Yeah, there are around $70$ possible candidates. But with described optimizations solution works much faster than $2^{70}$.
 » 12 days ago, # |   +16 How can I get an Open Cup login QwQ
•  » » 12 days ago, # ^ |   +3 Contact snarknews~
 » 12 days ago, # |   +20 I think the first step of A can be done by removing 3-degree vertex repeatedly.
•  » » 12 days ago, # ^ |   +40 Well, if you will delete arbitrary vertex of degree 3 it won't work, because at some moment you can delete some border vertex of the triangle, and there may be no possible variant to continue this decomposition :)
•  » » 12 days ago, # ^ |   +20 Depends on what kind of decomposition you're aiming for. We deleted vertices with degree 3 whose neighbourhood is a clique, but we can get some different decomposition than one described in statement (because of what ~300iq said), but it didn't matter to us at all. Maybe it is important to some other solutions.
•  » » » 12 days ago, # ^ | ← Rev. 3 →   +20 Yes, it was not correct way for "Apollonian Network". However it works when we ignore planarity and we can obtain a tree decomposition of tree-width -4- 3 in O(N). Then we must be able to construct O(N) DP.
•  » » » » 12 days ago, # ^ |   +30 Exactly, that's what we did during contest :)
•  » » » » 11 days ago, # ^ |   0 What's "tree decomposition of tree width 4"?
•  » » » » » 11 days ago, # ^ |   +10 For Apollonian Network you can build tree decomposition with treewidth 4
•  » » » » » » 11 days ago, # ^ |   +40 To be precise, bags are of size <=4, so the treewidth is <=3 xD
•  » » » » » » » 8 days ago, # ^ |   0 For dp part, what are the states for each bag (k4)? I feel there are too many (like if the corners are used, if the edges are used). But then merging three such states to get the parents state is yet another nightmare. Any simpler way to think about it?
•  » » » » » » » » 8 days ago, # ^ | ← Rev. 2 →   0 Actually you don't need info about edges, just maintain connected components of its vertices and their degrees :)Key idea is that in fact you can look at path like a "connected set of vertices with degrees <= 2 and exactly two vertices with degree = 1".
 » 12 days ago, # |   +9 300iq Can you share the problem set here as pdf or something for ppl without Open cup logins please :)
•  » » 12 days ago, # ^ |   +29 Soon I will load the contest to codeforces gym, don't worry.
 » 12 days ago, # |   +30 It's funny that problems H and J were kinda similar and during competition I thought that H will be solved by augmenting paths and J will be solved by parametric search but it turned out to be exactly opposite xD
•  » » 12 days ago, # ^ |   0 XD, for me it looks impossible to use augmenting paths, when you have queries on segments? :P
•  » » » 12 days ago, # ^ |   0 That's why I had no idea how to solve it xD. By the way, augmenting paths looked impossible in J as well, I have to admit that none of us understands anything from that solution even now :(
•  » » » » 12 days ago, # ^ |   +8 Oh, that is sad, I thought that this solution should be simple to come up with but impossible to write :)
•  » » » » 12 days ago, # ^ |   -28 I already told you swistakk, as the age passes you are not as clever as you were in your young days, so don't repent if any problem is not solved.
 » 11 days ago, # |   +13 In problem E how to do Gaussian Elimination in $O\left(\frac{(n+60)^2}{64}\right)$ per every new vector?
•  » » 11 days ago, # ^ | ← Rev. 3 →   +20 Store the basis in form $base[i]$ — the vector of the maximum price which has its first bit set at $i$ or None. When adding, iterate over the set bits $i$ of the new vector. As usual, if $base[i]$ is None, put it there and break. If the price of $base[i]$ is lower than the new one, swap your vector with $base[i]$ and continue as if the new vector was in the basis and you were adding $base[i]$ to it.
 » 10 days ago, # |   +28 Could someone please shed some more light on problem C? I don't really get the idea from the editorial.
 » 10 days ago, # |   +3 Can anyone give me 8th test of the problem D, I can't find my bug :(
•  » » 9 days ago, # ^ |   +8 You may write stress-test, it is not very hard in this problem.
•  » » » 8 days ago, # ^ | ← Rev. 2 →   0 I wrote stress-test and found my mistake after my comment :) Thanks anyway!