I discovered https://highload.fun the other day and it is my new obsession. My first naive solution here scored 309,783. After a day or two of finding ways to shave off execution time I finally claimed the top spot on the list.
I might try them in the other languages, but honestly C# is a challenge that I like. Especially as the site (currently) doesn't support unsafe code. Naturally it means the other three are far more competitive and have much lower scores.
As much fun as the site is I really do feel the need to call out the blurb on the front page:
Join the community of the best software engineers and write the fastest code.
Being able to micro-optimize contrived problems does not mean you are a good software engineer, let alone “best”. It’s a fun problem solving skill and shows a level of knowledge in the area but software engineering is so much more than that.