G4G0-2/Data Structures/GPT Answers to Past Paper/Question 7.md

Compare and contrast a queue with a stack.

Comparison of Queue and Stack

Aspect	Stack	Queue
Definition	A stack is a collection of elements that follows the Last-In, First-Out (LIFO) principle: the last element added is the first one removed.	A queue is a collection of elements that follows the First-In, First-Out (FIFO) principle: the first element added is the first one removed.
Primary Operations	- Push: Add an element to the top of the stack. - Pop: Remove the top element of the stack. - Peek/Top: View the top element without removing it.	- Enqueue: Add an element to the back of the queue. - Dequeue: Remove the element from the front of the queue. - Front: View the element at the front without removing it.
Order of Removal	LIFO: Last element inserted is the first to be removed.	FIFO: First element inserted is the first to be removed.
Structure	Operates on a single end of the collection (top).	Operates on both ends: elements are added at the back and removed from the front.
Real-World Analogy	A stack of plates: the last plate added is the first one removed.	A line (queue) at a ticket counter: the first person in line is the first one served.
Applications	- Function call stack - Undo operations in software - Backtracking algorithms (e.g., maze solving) - Expression evaluation and parsing	- Task scheduling (e.g., printer queue) - Breadth-first search (BFS) in graphs - Data buffers (e.g., IO buffers) - Handling requests in web servers
Performance	Both push and pop are O(1)O(1)O(1) operations in most implementations.	Both enqueue and dequeue are O(1)O(1)O(1) operations in most implementations.
Types/Variants	- Simple Stack - Double Stack - Min/Max Stack (keeps track of min/max values)	- Simple Queue - Circular Queue - Priority Queue (elements removed based on priority, not order) - Deque (Double-Ended Queue)

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Key Differences

Aspect	Stack	Queue
Access Pattern	LIFO (Last-In, First-Out)	FIFO (First-In, First-Out)
Addition (Insert)	Always at the top of the stack.	Always at the back of the queue.
Removal (Delete)	Always from the top of the stack.	Always from the front of the queue.
Flexibility in Ends	Operates only on one end (top).	Operates on both ends (front and back).

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Real-World Examples

Stack Examples:

1. Undo Feature in Text Editors:
   • When you perform actions, they are stored in a stack. Undoing removes the last action first.
2. Browser Back Button:
   • Tracks visited pages in a stack; the last page visited is the first to be returned when you press "Back."

Queue Examples:

1. Ticket Counter:
   • People stand in line (queue), and the first person in line is served first.
2. Task Scheduling:
   • Jobs sent to a printer are handled in the order they are received (FIFO).

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Summary:

• A stack is suitable for tasks where the most recent action needs to be undone or processed first (LIFO).
• A queue is ideal for tasks where the first action needs to be processed first (FIFO).