What is the fundamental difference between a queue and a stack?

The fundamental difference between a queue and a stack lies in the order of access to elements:

• Queue: Operates on the principle "First In — First Out" (FIFO). Elements are added to the end of the queue (enqueue operation) and removed from the front (dequeue operation).
• Stack: Operates on the principle "Last In — First Out" (LIFO). Elements are added to the top of the stack (push operation) and also removed from the top (pop operation).

A queue can be imagined as a line in a store, where the first person in line is served first. A stack can be imagined as a stack of plates, where the last plate placed on top is the first to be taken off.

Here is a comparative table:

Characteristic	Queue	Stack
Operation principle	FIFO (First-In, First-Out)	LIFO (Last-In, First-Out)
Operations	enqueue (add to end), dequeue (remove from front)	push (add to top), pop (remove from top)
Insertion point	End	Top
Removal point	Front	Top

Examples of usage:

• Queue: task scheduling in an operating system, handling requests on a server, input-output buffering.
• Stack: function calls (call stack), expression syntax analysis, browser history.