When to Use Sets

Use sets when you need:

$1.$ Fast membership checks ($O(1)$ average)

$2.$ Automatic duplicate removal

$3.$ Union, intersection, or difference operations

$4.$ Problems about unique elements or grouping

If the problem asks about common elements, missing elements, or unique elements, think sets first. Sets turn many $O(n^2)$ brute-force solutions into $O(n)$ solutions.