I've always tried to solve questions and upsolve 1 question that I wasn't able to in the order of difficulty. One thing is, I've never sat down and formally learn topics in depth and tried out questions. I've always relied on editorials, and blogs to explore the topic. Are there any courses, books, and/or blogs that can help improve in topics, let's say, Combinatorics(counting problems) that are more CP focused?
PS. Discrete Mathematics is something I have started with, maybe you can share how you went about it.
Thanks in advance.