6.3 Queues & Stacks

Let’s go through Queues and Stacks in C# — two fundamental data structures in programming.

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📌 1. Stack

A Stack is a Last-In, First-Out (LIFO) collection.

• Last element added is the first to be removed

• Common operations: Push, Pop, Peek

• Part of System.Collections.Generic

Example

using System;
using System.Collections.Generic;
 
Stack<int> stack = new Stack<int>();
 
// Add elements
stack.Push(10);
stack.Push(20);
stack.Push(30);
 
// Access top element without removing
Console.WriteLine(stack.Peek()); // 30
 
// Remove top element
Console.WriteLine(stack.Pop());  // 30
Console.WriteLine(stack.Pop());  // 20
 
// Remaining element
Console.WriteLine(stack.Peek()); // 10

Key Methods

• Push(item) → Add item to top

• Pop() → Remove and return top item

• Peek() → Return top item without removing

• Count → Number of elements

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📌 2. Queue

A Queue is a First-In, First-Out (FIFO) collection.

• First element added is the first to be removed

• Common operations: Enqueue, Dequeue, Peek

• Part of System.Collections.Generic

Example

Queue<string> queue = new Queue<string>();
 
// Add elements
queue.Enqueue("Alice");
queue.Enqueue("Bob");
queue.Enqueue("Charlie");
 
// Access front element without removing
Console.WriteLine(queue.Peek()); // Alice
 
// Remove front element
Console.WriteLine(queue.Dequeue()); // Alice
Console.WriteLine(queue.Dequeue()); // Bob
 
// Remaining element
Console.WriteLine(queue.Peek()); // Charlie

Key Methods

• Enqueue(item) → Add item to end

• Dequeue() → Remove and return item from front

• Peek() → Return front item without removing

• Count → Number of elements

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📌 3. Comparison Table

Feature	Stack	Queue
Order	LIFO (Last In, First Out)	FIFO (First In, First Out)
Main Operations	Push, Pop, Peek	Enqueue, Dequeue, Peek
Use Case Example	Undo/Redo, Browser History	Print Queue, Task Scheduling

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✅ Tips

• Use Stack for backtracking algorithms (undo, recursion, parsing)

• Use Queue for processing tasks in order (FIFO scheduling, BFS in graphs)

• Both are generic in C# and part of System.Collections.Generic