If this was the page that triggered the takedown, any copying was probably in the reverse direction (that is, the HackerRank material copied something from the documentation, which is legal, and their idiot copyright enforcer doesn't understand that).
It says a lot about the quality of Hackerrank as a company if they admit that the readme for an open source project contains the coveted solutions to their problems.
I am curious also here about opensource. What was the license if any?
Also, which license allows you to copy (and modify) for your own use, whether personal or not to reproduce?