I still having a hard time understanding the use for this after the job interview. The few times I’ve had to deal with data that was large enough to be big O and needed to optimize it, I just spend some time researching it a bit. Plenty of answers online for those cases. Someone who actually enjoys thinking and researching this subject, but not I’m not going to grind hackerrank.
It actually stopped me for even considering to apply at a company that uses this as a bar. A developer generally is someone who solves business problems, sometimes with code.
I’m happy that I work in a country where this grinding is not the norm.
Otherwise buy a $100 android and use the free app. Or use the windows and Linux versions (free too)