TCO19 Algorithm Round 3B and Parallel Rated Round is scheduled to start at 21:00 UTC -4 on July 18, 2019. Registration is open for the Round in the Arena or Applet and it closes 5 minutes before the match begins, so make sure that you are all ready to go.

Click here to see what time it starts in your area.

Algorithm Round 3 Qualified Competitors (https://tco19.topcoder.com/algorithm/round-2a and https://tco19.topcoder.com/algorithm/round-2b) are eligible to compete in Round 3B. Those who have already qualified for Round 4 (https://tco19.topcoder.com/algorithm/round-4) from online stages and Round 3A (https://tco19.topcoder.com/algorithm/round-3) are not eligible to compete.

All contestants who will end up with a positive score will be awarded a TCO19 T-shirt. The t-shirt will be shipped after the TCO19 Finals to held in November.

All the best! See you in the arena!

Why not Saturday or Sunday?

Why do you give hope to all those, who can participate in 9 PM, if round 4 will be in normal time :(

I liked 450-point problem. We can prune like DFS of $$$10$$$-ary segment-tree.

I think we can prove that the time complexity is at most $$$O(log(n) \times f(n))$$$ where $$$f(n)$$$ is the time complexity of primality test, because all even numbers (except $$$2$$$) are composite.

The medium and hard were written by Errichto.

Your time complexity estimate is correct. Essentially, a prefix that is not composite must end with an odd digit, and changing that digit by 1 makes the prefix even and therefore composite. (There are some extra details to handle, as you all know, but this is the main idea of the time complexity argument.)

I ended up with a positive score due to a blind challenge. Luckily, the first challenge worked, although just so that I can challenge more than once I submitted my 250 even when it wasn't passing all example cases.

The story doesn't end here. I advanced to this round also due to a blind challenge in the Round 2A.

Although it helped me in winning a t-shirt I think Codeforces challenge system is better than Topcoder's as it allows only those to challenge whose solution has passed the pretests. I think Topcoder should do the same.

This is like to compare an apple and an orange. You have to pass pretests and lock your problem on CF because you can challenge during contest time (and possibly using another account to submit other people's code). There is a separate challenge phase on TC so you're not required to pass pretests. If you submit a wrong code just to gain more challenge attempts, you also give a chance for people in your room to challenge your code.

How do you solve the third problem?

I've written a solution here.