### fvogel's blog

By fvogel, history, 13 months ago,

Hello Codeforces,

I couldn't find any blog on this year's round C so I decided to create one. Feel free to post your feedback and ideas on the contest.

Personally for me the start of the contest went pretty well. Problem D was particularly interesting and I solved it quickly. However I found problem C quite harder and got quite a few WAs. But I finally got a perfect score!

I plan on hosting a live discussion about the problems from this contest today at 6:00 PM UTC. The link will be posted here shortly. See you there!

Happy coding :)

UPD 2: There's been a bug, here is the new link: youtube.com/watch?v=SL1-YYocQik

• +15

| Write comment?
 » 13 months ago, # |   +15 Got stuck on C, went to D, copypasted dp, got AC, went back to C, started crying.Was C simulating it very fast or smth else?
•  » » 13 months ago, # ^ |   0 Only subtasks 1 and 2 can be solved with clever brute force
•  » » » 13 months ago, # ^ |   0 can you tell what's your approach?
•  » » » » 13 months ago, # ^ |   +3 For each ant, find all the ants before it moving to the right and those after it moving to the left. Then you can notice that when two ants meet, only their indices swap but everything else stays the same, so if you don't care about their indices it's fine to just simulate the process. So sort the left part in descending order and the right part in ascending order then you can just brute force the intersections in $O(n)$.
•  » » 13 months ago, # ^ |   0 How did you approach D? I knew it had to be dp but couldn't figure out transitions and other stuff. Was it a well known problem?
•  » » » 13 months ago, # ^ | ← Rev. 2 →   +11 Simply just find total number of subsequences in range 0 to n-1 of length k using dp where k varies from 0 to n-1 and just use PNC after that
•  » » » 13 months ago, # ^ |   +3 May be this will help https://codeforces.com/blog/entry/56448
•  » » 13 months ago, # ^ | ← Rev. 2 →   0 Can you solve the easier question: when will the last remaining ant fall off? The key observation from that problem is quite useful in C.
 » 13 months ago, # |   +5 I also solved D but didn't solve C.
 » 13 months ago, # |   0 Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).
 » 13 months ago, # | ← Rev. 2 →   0 unable to solve C, is that kind of bfs ?
•  » » 13 months ago, # ^ |   0 I used a queue to solve two subtasks.
•  » » 13 months ago, # ^ |   0 I used sorting, idea inspired by classic ant problem.
 » 13 months ago, # |   0 Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).
 » 13 months ago, # |   -8 I made Video Solutions for all problems in case someone is interested.
 » 13 months ago, # |   0 Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).
 » 11 months ago, # |   0 pls anyone give hint about problem c
 » 11 months ago, # |   0 Nice round
 » 11 months ago, # |   0 ya nice round