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:** Here is the link: https://www.youtube.com/watch?v=jvc_4ZT9da4

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

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?

Only subtasks 1 and 2 can be solved with clever brute force

can you tell what's your approach?

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 $$$O(n)$$$.

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?

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

May be this will help https://codeforces.com/blog/entry/56448

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.

I also solved D but didn't solve C.

Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).unable to solve C, is that kind of bfs ?

I used a queue to solve two subtasks.

I used sorting, idea inspired by classic ant problem.

Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).I made Video Solutions for all problems in case someone is interested.

Auto comment: topic has been updated by fvogel (previous revision, new revision, compare).pls anyone give hint about problem c

Nice round

ya nice round