What distinguishes a stack from a queue, and what are typical use cases?

• A. Stack is LIFO; queue is FIFO; stacks are used for function calls and undo; queues are used for task scheduling and buffering.
• B. Stack is FIFO; queue is LIFO; stacks are used for math operations; queues are used for sorting.
• C. Stack is used for query processing.
• D. Queues can only be used for function calls.

Answer: (A)

The main idea is the order in which elements leave the structure. A stack uses last-in, first-out: you push items on and the most recently added item is the first one removed. A queue uses first-in, first-out: items are removed in the same order they were added, from the front.

This order difference drives their common uses. Stacks are great for scenarios where you need to remember a sequence of operations to be reversed or unwound. In programming, the call stack holds function return addresses and local variables, so nested calls unwind correctly as functions return. Stacks also underpin undo features, where you reverse the most recent action first, and they show up in algorithms that explore possibilities in a last-visited-first way, like depth-first search or evaluating nested expressions.

Queues, by contrast, are ideal when processing items in the exact order they arrive. They’re used for task scheduling (so the oldest task runs first), print queues, and buffering data between producers and consumers (such as network packets or message queues) where you don’t want newer items to overtake older ones.

So the distinguishing factor is the removal order—LIFO for stacks and FIFO for queues—tied to practical use cases that align with that order.