Queue

Also known as: FIFO

A queue is a FIFO (first-in, first-out) collection: enqueue at the back, dequeue at the front. Ring buffers (arrays) and linked lists are common implementations.

BFS is the algorithms use case: the queue holds the frontier by discovery order. Task scheduling and IO buffers use the same idea.

Core operations are $O(1)$. A priority queue is not FIFO; it always extracts the best key instead.

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