Personnally my main practice has been to solve as many katas as I could, starting with the easiest ones. This way you practice basic things, increase your logic sense, and maybe the most important, you also learn some efficient techniques from other's solutions, and then get able to use them in different situations.
You can read about algorithmics too, it cannot be bad :)

With practice one sees better how to limit bruteforce approach and go to the strictly necessary.

Thank you so much for writing it down. I just wonder what you have to do to be able come up with such solutions. Read books? Solve 3000 katas? Read documentation?

what happened here? can someone explain please?