Worth mentioning that there's a companion book to this website: https://cses.fi/book/book.pdf

The problems are indeed of very high quality. But it can be a difficult place to start. For example, even the very first problem has an overflow gotcha built into it. Also, Kadane's Algorithm appears as an early problem even though several mathematicians and computer scientists failed to discover it: https://en.wikipedia.org/wiki/Maximum_subarray_problem#Histo...

Great resource, but it's hard to practice there as a beginner because there are no solutions (unless you solve the problem yourself). So you might have to get comfortable with being stuck. Which is fine when you're experienced, but will demotivate you if you're just starting.

Interesting that they have Assembly, but not C.

In practice it seems most people just use C++ (87.5%). Add Python3 (7.5%) and Java (5%) and 99% of submissions are accounted for.