Stack and queue — how do they differ and where are they used?

Junior PRO

The gist

• Stack — LIFO: push and pop at one end (push/pop). Uses: the call stack, undo, depth-first traversal (DFS), bracket matching, expression parsing, the navigation stack (NavigationStack, UINavigationController).
• Queue — FIFO: enqueue at the tail, dequeue from the head. Uses: task queues (DispatchQueue, OperationQueue), breadth-first traversal (BFS), event buffers.
• Deque — O(1) operations at both ends; not in the stdlib, but Deque exists in swift-collections.