Stacks vs Queues: When to Use Which

With both structures now built from scratch, this short chapter exists purely to crystallize the distinction between them before moving on. No new code beyond one quick reference to Python's standard library.

• -A browser-style back button, undo history, and matching parentheses all want a stack. The most recent action matters first.
• -A printer job queue, a customer support ticket line, and processing requests fairly in arrival order all want a queue.
• -Tracking in-progress function calls during recursion is a stack, whether or not it is managed by hand. This connects directly to the recursion module later in this tier.
• -Task scheduling and breadth-first search, covered in later tiers, both want a queue for the same fairness reason.

Python's standard library already ships a structure that behaves correctly as either one: collections.deque, a doubly-linked list under the hood offering O(1) append and pop from both ends. Building a Stack and a Queue from scratch in this module was not busywork. Understanding why list.pop(0) is slow, and how a circular buffer avoids that cost, is exactly the reasoning that lets you evaluate any structure's complexity, including ones a language or library hands you ready-made, or the constrained sandbox environment in the Forge Algorithm Lab where a full standard library is not always available.