Which textbook can I use after a high school CS course?
Personally, I recommend either Algorithms by Dasgupta, Papadimitriou, and Vazirani, or The Algorithm Design Manual by Skiena.

The Algorithm Design Manual is super-practical and full of excellent pragmatic advice, useful stories, and is well-written and easy to read.  If you want to help get ready for programming contests, I strongly recommend that one.

I think Dasgupta has a better introduction from the perspective of learning the conceptual material, the theory, the proofs underneath the algorithms, etc.  So, if you want to learn the theory, I recommend Dasgupta.

You won't go wrong with either; they are both outstanding.

Personally, I think they are better than Cormen.  I learned from Cormen, and it's a perfectly fine book, but it can be a bit dense in places and a bit challenging to learn from, so it wouldn't be my first recommendation.  Still, it's a reasonable contender and you wouldn't be led astray by it; I just think the others are better.