I've just entered Highschool in Vietnam, i really want to participate in Math Olympiad (National Olympiad, TST and maybe IMO even though I know it's hard). Functional equation and Polynomial are the topics that I have least knowledge so I want to find some good books for them.
I found "Awesome Polynomials for Mathematics Competitions" and "Topics in Functional Equations – 3rd Edition" by Titu Andresscu, but i don't know if they are worth reading. Do these books provide all the theories, techniques, skills for Olympiad? I want some reviews about them.
Also, what are some good books for these Topics, if the above two books are not good?