Start with the Internet Archive for old NUS Lecture Notes. Then move to the AoPS forums for community-shared past papers. Finally, invest in a PDF annotation tool to make those digital pages your own.
While originally international, the Singapore edition includes localized problem sets from the NUS High School training camp. singapore math olympiad books pdf
I can explain a specific math heuristic (like the "Gap and Difference" method). Start with the Internet Archive for old NUS Lecture Notes
Focuses on polynomials, inequalities, and functional equations. and functional equations.