## What is it? A Queue is a First-In-First-Out (FIFO) data structure. Elements are added (enqueued) at the rear and removed (dequeued) from the front. Think of a queue of people waiting in line. ## How it works **Core operations (all O(1) with proper implementation):** - **enqueue(x)** — add `x` to the rear - **dequeue()** — remove and return the front element - **peek() / front()** — return the front element without removing it - **isEmpty()** — check if the queue has no elements **Implementations:** - Circular array (avoids O(n) shifting on dequeue) - Linked list (push at tail, pop at head) - Two stacks (amortized O(1)) ## When to use - Breadth-First Search (BFS) - Task scheduling / print spooling - Sliding window maximum (with a deque) - Producer-consumer problems ## Key Points - FIFO — the first element added is the first removed - A Deque (double-ended queue) supports push/pop at both ends - Priority Queue orders elements by priority, not insertion order