I couldn't solve any of them but I liked boxes and horses the most. Also scales was very interesting.

Here are my unfiltered opinions on the tasks, and I will include some personal stories that relate to my assessment of the tasks:

boxes: I didn't understand why the limits were 10⁷. It seems like you tried to fail , but I passed with that anyway. It's an okay easy task, although I missed a really clever observation (only one circle trip is necessary) and wasted a lot of time on this. I heard it's a duplicate problem from somewhere very recently though, which is kind of unfortunate.

scales: My initial solution got 45.45. I then spent a lot of time writing another program to find a smaller set of queries to solve the problem, but it wasn't very good (it searched top-down instead of bottom-up) and it only found a solution with 8 queries, which was only about a 10 point improvement from my initial solution. That was really disappointing, but I didn't want to have wasted over an hour for nothing, so I tried (without success) to modify my solution to actually solve the problem. I didn't think I could debug the code and solve teams within the remaining time, so I gave up on the 10 points (it was really not worth it in hindsight). Anyway, I might have liked this task if the circumstances were different. I still like the idea of the solution though.

teams: I didn't have too much time to solve this task (because of the reasons above), but it had a lot of different ways you could approach it (only some of which lead to a good and fast solution). I came up with some solution (not the one with Hall's marriage theorem) in and coded and submitted it, but it turned out that there were too many bugs to count and fix before the contest ended (As the graders had a massive queue and I was pretty confident in this solution, I started to relax, but decided to write a maxtest anyway and found my code was wrong). Also, while debugging this problem near the end of the contest, I spilled water all over my desk and then CodeBlocks IDE froze my whole laptop while in debug mode, so after these two events I knew there was no way I would finish this problem in time. This had a really big impact on my enjoyment of the task, so I ended up not liking it that much (more so when I heard one of the fast solutions is just to optimize the 34 point solution).

horses: I thought this one was pretty standard — a greedy idea optimized with segtrees (just take logarithms and use doubles). I was 100% sure precision wouldn't be an issue when I coded this. After the contest while discussing solutions, another participant (who wishes to remain anonymous) seemed to think that this solution was really satanic, and told me a "better" solution that used integers only. Anyway, I think it fulfilled its purpose as the easiest task pretty well, although I know for sure some others disagree with me.

sorting: First, I want to say that I prefer "standard" OI-style tasks over interactive-type problems. Many participants found this task easy (and boring), but this is personally my favorite task of IOI 2015. The initial impression of this problem and your perspective were very important in how easy solving this task was. During the contest, I was thinking about some solution involving cycles in the permutation, but it was really complicated and I wasn't confident I could implement it in time. After the contest, I learned of the nice interpretation where the first player is swapping indices and the second player is swapping values, which was really cool and made it obvious you could consider the two players separately. I'm still pretty salty about not seeing this during the contest after spending a good amount of time thinking about it, but I still think the solution is cool.

towns: This task was difficult for me to get a good score. The time spent versus points earned ratio on this problem was really low for me, and even the first subtask wasn't really trivial like the other problems. Kuroba wanted a nonzero score on every problem, but ran out of time before getting any points on this problem (which affected his points earned on sorting, a sad story indeed). I recalled the majority algorithm during the contest, but since I never implemented it I forgot it works even if you don't know the exact candidates. I know for some this problem is their favorite, but to me it feels really artificial — it's just a combination of two easier tasks to form a much harder task (optimize majority algorithm + check if a hub lies on the path between two leaves). Anyway, the main point is that I thought the subtask scoring was harsh — a lot of work for very little improvement in score, at least for me.

On an semi-unrelated note, is there any chance that IOI will get a live scoreboard for the contestants? Having a live scoreboard would change my (really bad) strategy a lot. I might have done better if I chose a proper order for solving (1-2-3 and 1-3-2 on the two days, based on a initial 10 second feeling), but spending time to assess the difficulty of a problem is risky (since you may not get any new ideas after all).

Towns was certainly the best one for me. I liked it a lot and we came up with solution idea just 10 hours after the contest! Scales and teams were also very cool problems. Having said that I solved neither of those three I mentioned, but I still think they were great (costed quite alot of push-ups though)

On a side note, will author solutions be posted?

I've solved the boxes first, it was pretty simple one. After then started think about teams. Come up with the idea that M < 1000, implemented a solution and expected to get 77 points. It turned out this solution was able to get 100 from the test data. Here comes the sad part. There was 2.5 hours left for scales. I tried to implement some backtracking algorithm. After 1.5 hours of debugging and implementing I realized that I would not be able to finish it in time. Then I implemented quick sort to see how much points it would get. Because of stupid bugs and slow feedback could able to implemented in an hour. Anyways, end up with 26.5 points which is much less than selection sort.

Second day first problem was easy as well, just like boxes, I come up with with an idea that uses keeping answer in log and lazy propagation. After 1 hour I got accepted.

Towns was a non-standart problem, that's why I decided to go for sortings. I thought about cycle permutations for 30-40 minutes, but could not come up with something alltough I was pretty sure that this problem was about cycle permutations :(. Then I've decided to try towns. First thing that came to mind that center of the tree has to be laying on diameter of the tree. And for every node u fartest node to u should be one of the ends of the diameter. It's is quite easy to find the diameter with 2 * N queries. And for deciding if center is balanced or not, we need to be able to answer if any two nodes are in the same subtree or not. If center of the tree is C then A, B are in the same subtree if and only if dist(A, C) + dist(C, B) > dist(A, B). I forgot that there could be two centers, and thought that I had some bug in the implementation, it was just 13 points more(35 to 48) so I decided to go for sorting again and let it go.

I thought about a solution that tries to put numbers each time to their place with other person's swaps but I was not sure that it guarantees the solution if there is one. Then I implemented O(NM) solution to see if it passes the subtask with N ≤ 500. It got wrong answer from just 1 test case. I thought may be test cases were a bit week and my solution was completely wrong:(. It turns out my bug was I forgot to add swaps that does not change anything after guaranteeing that every number will be placed with second person's moves. That's why left with 36 points and silver medal. And then I turned back to towns and realized that there could be two centers got more 13 points.

Without going into details, my overall impression is that the problemset is much better than 2010-2014 (2010 and 2013 were especially horrible), but still not so goddamn hot as it used to be in 2007-2009.

Where can I read the problems and submit solutions? Please someone tell me.