Explain the Difference Between Stack and Queue Data Structures

Concept

Stacks and Queues are linear data structures used to store and manage elements in a particular order.
They differ in how elements are added and removed:

Stack: Follows Last In, First Out (LIFO) order.
Queue: Follows First In, First Out (FIFO) order.

These structures are fundamental in program execution, resource scheduling, and algorithm design.

1. Stack — LIFO Structure

A Stack works like a pile of plates — the last one placed is the first removed.

Operations:

push() → Add item to the top
pop() → Remove top item
peek() → View top item without removing it

Example (safe for MDX):

Stack: [1, 2, 3]
push(4) → [1, 2, 3, 4]
pop() → [1, 2, 3]

Common Use Cases:

Function call stack in recursion
Undo/redo features in editors
Expression evaluation (e.g., postfix, prefix notation)
Backtracking algorithms (e.g., DFS)

2. Queue — FIFO Structure

A Queue behaves like a waiting line — the first person in line is served first.

Operations:

enqueue() → Add item at the rear
dequeue() → Remove item from the front
front() → View the front item

Example (safe for MDX):

Queue: [1, 2, 3]
enqueue(4) → [1, 2, 3, 4]
dequeue() → [2, 3, 4]

Common Use Cases:

Task scheduling (CPU, I/O operations)
Print queues
Breadth-First Search (BFS) in graphs
Message buffers and producer-consumer systems

3. Comparison Table

Aspect	Stack	Queue
Order	LIFO (Last In, First Out)	FIFO (First In, First Out)
Access Point	Top only	Front and rear
Implementation	Array or Linked List	Array or Linked List
Key Operations	Push, Pop	Enqueue, Dequeue
Real-World Analogy	Stack of plates	Waiting line
Algorithm Usage	DFS, backtracking	BFS, task scheduling

4. Variants

Structure	Description
Deque (Double-Ended Queue)	Allows insertion/removal from both ends.
Priority Queue	Elements dequeued based on priority, not order.
Circular Queue	Optimized version that wraps around array boundaries.

5. Implementation Notes

Stack (Array-based):

push(x):
  top += 1
  stack[top] = x

pop():
  if top >= 0:
    top -= 1

Queue (Linked List-based):

enqueue(x):
  rear.next = x
  rear = x

dequeue():
  front = front.next

6. Performance Summary

Operation	Stack	Queue
Insert	O(1)	O(1)
Delete	O(1)	O(1)
Peek	O(1)	O(1)
Space	O(n)	O(n)

Both are extremely efficient due to constant-time operations.

7. Real-World Examples

Scenario	Data Structure
Undo in Word Processors	Stack
Web Browser Back/Forward Navigation	Stack
Print Job Management	Queue
Task Scheduling in OS	Queue
Customer Service Ticketing System	Queue

8. Interview Tip

Explain order behavior clearly (LIFO vs FIFO).
Mention real-world analogies to make your answer relatable.
Know how to implement both using arrays and linked lists.
Discuss variations like priority queues or deques if prompted.

Summary Insight

Stacks remember the past; queues manage the future. Together, they form the backbone of algorithmic control and execution flow.