CS205Data Structures

Stacks & Queues

Fundamental linear data structures with different access patterns

📚Stack (LIFO)

Last-In, First-Out

Think of a stack of plates. You can only add or remove from the top. The last plate you put on is the first one you take off.

3 ← top

2

1

Operations: push (add to top), pop (remove from top), peek (view top)

🚶‍♂️Queue (FIFO)

First-In, First-Out

Think of a line at a store. The first person in line is the first to be served. New people join at the back.

front →

1

2

3

← rear

Operations: enqueue (add to rear), dequeue (remove from front), peek (view front)

When to Use Which?

Use a Stack when:

🔙Undo/Redo - Most recent action should be undone first
🌐Browser History - Back button goes to most recent page
📞Function Calls - Most recent function returns first
🔤Balanced Parentheses - Match most recent opening bracket
🔄Reverse Order - Need to process in reverse

Use a Queue when:

🖨️Print Queue - Documents print in order sent
⏳Task Scheduling - First task submitted runs first
🌳BFS Traversal - Explore level by level
🎫Customer Service - First come, first served
📨Message Buffers - Process messages in order

Quick Comparison

Aspect	Stack	Queue
Order	LIFO (Last-In, First-Out)	FIFO (First-In, First-Out)
Insert	push() - at top	enqueue() - at rear
Remove	pop() - from top	dequeue() - from front
View	peek() - top element	peek() - front element
Time Complexity	O(1) for all operations	O(1) for all operations
Java Class	Stack<E> or Deque<E>	Queue<E>, LinkedList<E>

Explore Topics

Stack Deep Dive

Interactive stack operations, balanced parentheses, and more

• Drag-and-drop push/pop
• Balanced parentheses checker
• Array vs LinkedList implementation

Queue Deep Dive

Interactive queue operations and priority queue concept

• Drag-and-drop enqueue/dequeue
• Priority Queue (ER triage demo)
• Array vs LinkedList implementation

Side-by-side comparison and decision guide

• Same operations, different results
• Implementation comparison
• When to use which