Sliding Window Mastery

Almost every sliding window problem lives or dies on one question you answer in the first ten seconds: is the window a fixed size, or does it grow and shrink? Answer it right and the template basically writes itself. Answer it wrong, or fail to spot that you're even looking at a window problem, and you'll burn twenty minutes reinventing an O(n²) loop.

Two questions settle it, and neither takes more than a glance.