A recruiter contacted me after I submitted my resume at a conference.

The recruiter's emails were timely, friendly, and very straightforward. I was first given a practice HackerRank problem and was then invited to solve an actual HackerRank challenge. I was given four days to start the challenge and 75 minutes to complete it. I think the problem was very reasonable, although I would have preferred to have more time.

Unfortunately, I did not do well enough on the coding challenge, so I was unable to move forward to the next stage. However, even the rejection email was very friendly and supportive, so overall the recruitment process left a really good impression on me.

A very reasonable HackerRank problem.

I would suggest trying out a few problems on the site beforehand to get used to the way HackerRank works. The challenge link does give you a brief tutorial, but it might be better to try it out in a no-pressure setting.

I would say that the interview problem was a medium-difficulty problem, perhaps made a bit harder by the time limit.