What is a queue and where is it used?

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A **queue** is a **FIFO** (First-In, First-Out) collection: elements are added at the **back** and removed from the **front**, like a line of people. ## Operations ```text enqueue(1) enqueue(2) enqueue(3) dequeue()->1 front ->[1][2][3]<- back front ->[2][3]<- back ``` ## Example ```python from collections import deque queue = deque() queue.append('a') # enqueue (back) queue.append('b') first = queue.popleft() # dequeue (front) -> 'a' ``` A plain Python `list` makes `pop(0)` an **O(n)** operation, so use `deque` for **O(1)** at both ends. | Operation | Time (deque) | |---|---| | enqueue | O(1) | | dequeue | O(1) | | peek front | O(1) | ## Common uses - **Task / job scheduling** — process work in arrival order. - **Breadth-first search (BFS)** — visit nodes level by level. - **Buffering** — producer/consumer pipelines, print spoolers, request queues. ## Why it matters Queues enforce fairness and ordering, which is critical for schedulers, messaging systems, and BFS. Whenever work must be handled "in the order it arrived," a queue is the natural fit.