I recently study FFT and was eager to practice some simple FFT problems with my new knowledge. Thus, I visit the link https://codeforces.net/problemset/tags/fft?order=BY_SOLVED_DESC, which supposedly have FFT, and I try to find easier problems by number solves.

I look at the first few problems and, for the life of me cannot figure out how to do with new algorithm. Of course, I consult book and article and review my concept but on closer inspect I am not longer positive this is my issue rather than the website: there is peculiar fact that four of top 5 problems are from same contest!

No way that there were many FFT on one contest! I think and on looking at the other tags and attempt without fixing mindset of FFT, those appear to correct and FFT be misplace than the rest.

There does not appear to issue with any other tags, so I ask, what is up with FFT?

And if it truly is silly blue me's fault not able applying FFT to these simple problems, I ask to hint, how to do so?

Update: Seems issue has been fixed, and problem tag FFT has been removed. Thanks!