Advent of Code in Idiomatic Kotlin will give you practice in efficiently solving problems with Kotlin. I haven’t watched these (except for the first one of the 2024 series), but they were recommended by a student from last year. One caveat here is that the examples use idiomatic solutions, which means that they prioritize using language-specific conventions and tricks to solve problems rather than using the most straightforward methods. While this approach is nice to level-up your Kotlin skills, don’t be intimidated that you would have used a different strategy to solve the problems.