Collections with no duplicates Day 135 of 149 👉 Full deep-dive with code examples The Guest List Analogy You're making a party guest list: Add "Alice" → Alice is on the list Add "Bob" → Bob is on the list Add "Alice" again → Nothing happens! She's already there Sets work like guest lists! Every item appears exactly once. Duplicates are automatically ignored. The Problem They Solve Sometimes you need unique items: "What unique words are in this document?" "Who has visited this page?" "Which tags are used in this article?" With arrays, you'd have to check for duplicates yourself. Sets handle it automatically! What Sets Can Do Membership check (fast!): "Is Alice on the list?" → Yes/No instantly Add items: Adds if not there Ignores if already present Remove items: Takes the item out Set operations: Union: Combine two sets Intersection: What's in BOTH sets? Difference: What's in one but not the other?…