I think (or at least hope) that copyright is limited to a specific expression of a specific solution. Anything more broad would be tantamount to a copyright on basic algorithms, I think.
And what happens when there are multiple different possibilities for the solution? As is the case here, the solution can be achieved in multiple ways. It would be bizarre for one to have copyright claim over all of them.
My initial reaction to your question was that I thought perhaps Hackerrank could claim copyright on any particular solution that had either been written by Hackerrank or where the copyright had been assigned to it.
With regard to the latter, I would guess, knowing how corporations work, that Hackerrank requires anyone taking a test to assign her rights, with respect to any solution given, to the company. (To be clear: I do not like that at all.)
In the US, the Copyright Act of 1976 extended copyright to unpublished works (Hackerrank presumably does not publish its own solutions, though it might well register them.) IANAL and AFAIK, I think there are fairly stringent requirements on fidelity for a work to be considered infringing, and there is also the matter of fair use, but in practice in this case, it is Github, not the courts, that Hackerrank has to persuade.
I would guess that publishing something that advertises itself as a solution to a Hackerrrank problem might fall under trademark infringement or some such law, and something stating the particular problem being being solved might be an infringement of a copyright on the problem as stated by Hackerrank.
> I would guess that publishing something that advertises itself as a solution to a Hackerrrank problem might fall under trademark infringement or some such law,
Trade secret, rather. In fact, given the questions are supposed to be unpublished, those would more properly fall under trade secret law as well (except that the DMCA doesn't apply to trade secrets or any other form of IP other than copyright).