How do a stack and a queue differ, and where would each be useful?

A stack removes the most recently added item first, making it useful for undo history, expression evaluation, and iterative depth-first traversal. A queue removes the oldest item first, making it useful for breadth-first traversal and first-come processing. Choose operations that are efficient in the selected language. For example, removing the first element of a Python list shifts remaining elements, whereas a deque supports efficient operations at both ends. Real work queues also need policies for failures, duplicate delivery, and capacity.