Very closely followed the book
I read the book over 40 years ago and remembered some things previewing but as the movie played, I realized it was following the book and all the action word for word. I've never seen a movie almost the same as the book itself. I only say almost because I have not reread the book to check for any discrepancies.
I imagine that because the book wasn't very long, the content fit quite nicely within the time limits of the movie.