The interview process consisted of me interviewing over the phone with a company called Karat, which is a contractor for Roblox. On their website, Karat prides themselves with letting you know the criteria they use to judge you when they interview. The only problem is that the actual decision of wether you move on or not in the interview process is not Karat's, is Roblox, so as a result you've got two companies with different criteria deciding wether you are good programmer or not, and this is confusing. I managed to finish my programming challenge correctly, (verified by my interviewer), which is Karat's most important criteria, and still failed. Was the code clean? nope, I ran out of time for this. I focused on what Karat wanted to see from me, but not Roblox. So, my advice, try as best as you can to find out what is Roblox's criteria, as well as Karat's, and do your best to impress them both. It's hard enough to impress one company, but in this case, you've got two of them.
For my c++ programmers, the challenging part is not so much coming up with an algorithm, is more about wether you remember STL syntax. This is the sad part of this process, you can get rejected over something that is trivial to do on a normal job, but during the interview, you can't spend much time looking at Google because you'll run out. This is where I struggled the most, STL is a BIG library, and if you don't use it everyday you won't remember it. Just go over the more popular containers, know how to iterate through the elements, and hope you don't get template errors, because diagnostic window will just dump lots of useless info at you. Focus on what's on the top. And lastly, check if the editor has autocomplete, it usually does, and it will help you remember. If you don't have to use C++, don't, python would be excellent with these tasks because the language is just so much easier.
Good luck.