They also seem to claim copyright on renditions of "their" questions in various repositories containing interview notes in general, e.g. https://github.com/jayshah19949596/CodingInterviews/blob/mas...
There's also a number of URLs in here which have "leetcode" and not "hackerrank" in them.
A sibling points out this golden nugget from the DMCA notice: "(the whole repository is infringing copyright as it contains the solution)" i.e. HackerRank is claiming that not only they own the copyright on "How to find the first occurrence of x in s?" but also every solution to that question, which is a completely ridiculous notion.