Stacks and Queues

📚 Stacks and Queues: The Story of Two Lines

🎭 The Tale of Two Worlds

Imagine you’re at a busy restaurant. In the kitchen, there’s a stack of plates. The cook puts clean plates on top, and when the waiter needs one, they take from the top too. That’s a Stack!

Now look at the customers waiting outside. The first person who arrived gets seated first. New people join at the back. That’s a Queue!

These two simple ideas power everything from your browser’s “Back” button to how your phone handles messages!

🥞 Chapter 1: The Stack — A Pile of Pancakes

What is a Stack?

Think of a stack of pancakes on your plate:

• You add new pancakes on top
• You eat from the top first
• You can’t grab a pancake from the middle!

This is called LIFO: Last In, First Out

The last pancake added is the first one you eat!

graph TD
    A["🥞 Pancake 3 - TOP"] --> B["🥞 Pancake 2"]
    B --> C["🥞 Pancake 1 - BOTTOM"]
    style A fill:#FFD700
    style B fill:#FFA500
    style C fill:#FF6B6B

Real Life Examples

• 📚 A stack of books on your desk
• 🍽️ Plates in a cafeteria
• ↩️ The “Undo” button in apps
• 🔙 Your browser’s “Back” button

🎯 Chapter 2: Stack Operations

A stack has four magical powers:

1. PUSH — Add to the Top! 🔼

// Adding an item to stack
stack[++top] = newItem;

Like putting a new book on top of your pile!

2. POP — Remove from Top! 🔽

// Removing top item
item = stack[top--];

Like taking the top book off your pile!

3. PEEK (or TOP) — Look Without Touching! 👀

// Just looking at top item
topItem = stack[top];

Like checking which book is on top without moving it!

4. isEmpty — Is the Stack Empty? 🤔

// Check if stack is empty
if (top == -1) {
    // Stack is empty!
}

Like checking if your plate has any pancakes left!

🏗️ Building a Stack in C

Here’s a simple stack in C:

#define MAX 100
int stack[MAX];
int top = -1;

void push(int x) {
    if (top < MAX - 1) {
        stack[++top] = x;
    }
}

int pop() {
    if (top >= 0) {
        return stack[top--];
    }
    return -1;
}

int peek() {
    if (top >= 0) {
        return stack[top];
    }
    return -1;
}

int isEmpty() {
    return top == -1;
}

Watch It Work!

graph LR
    A["push 10"] --> B["[10]"]
    B --> C["push 20"]
    C --> D["[10,20]"]
    D --> E["push 30"]
    E --> F["[10,20,30]"]
    F --> G["pop"]
    G --> H["[10,20]"]

🚶 Chapter 3: The Queue — A Line at the Movies

What is a Queue?

Picture the line at a movie theater:

• New people join at the back
• People enter the theater from the front
• No cutting in line!

This is called FIFO: First In, First Out

The first person to arrive is the first to be served!

graph LR
    A["🧍 Person 1 - FRONT"] --> B["🧍 Person 2"]
    B --> C["🧍 Person 3"]
    C --> D["🧍 Person 4 - REAR"]
    style A fill:#4ECDC4
    style D fill:#FF6B6B

Real Life Examples

• 🎬 Line at movie theater
• 🖨️ Print jobs waiting to print
• 📱 Messages waiting to be sent
• 🎮 Players waiting to join a game

🎯 Chapter 4: Queue Operations

A queue has four super powers:

1. ENQUEUE — Join the Line! ➡️

// Adding to rear of queue
queue[++rear] = newItem;

Like joining the back of a line!

2. DEQUEUE — Leave from Front! ⬅️

// Removing from front
item = queue[front++];

Like the first person being called forward!

3. FRONT (or PEEK) — Who’s First? 👀

// Looking at front person
firstItem = queue[front];

Like seeing who’s at the front without moving them!

4. isEmpty — Is the Line Empty? 🤔

// Check if queue is empty
if (front > rear) {
    // Queue is empty!
}

Like checking if anyone is still waiting!

🏗️ Building a Queue in C

Here’s a simple queue in C:

#define MAX 100
int queue[MAX];
int front = 0;
int rear = -1;

void enqueue(int x) {
    if (rear < MAX - 1) {
        queue[++rear] = x;
    }
}

int dequeue() {
    if (front <= rear) {
        return queue[front++];
    }
    return -1;
}

int peek() {
    if (front <= rear) {
        return queue[front];
    }
    return -1;
}

int isEmpty() {
    return front > rear;
}

Watch It Work!

graph LR
    A["enqueue 10"] --> B["[10]"]
    B --> C["enqueue 20"]
    C --> D["[10,20]"]
    D --> E["enqueue 30"]
    E --> F["[10,20,30]"]
    F --> G["dequeue"]
    G --> H["[20,30]"]

⚔️ Stack vs Queue: The Ultimate Showdown

🌟 Why Do These Matter?

Stacks Power:

• ↩️ Undo/Redo in apps
• 🧮 Calculators - solving math expressions
• 🔙 Browser history - going back to previous pages
• 🔄 Function calls - how programs remember where to return

Queues Power:

• 🖨️ Print queues - documents waiting to print
• 📬 Message systems - texts waiting to send
• 🎮 Game lobbies - players waiting for matches
• 🌐 Web servers - handling requests in order

🎉 You Did It!

You just learned two of the most powerful ideas in programming:

1. Stacks = Last In, First Out (like pancakes!)
2. Queues = First In, First Out (like a movie line!)

These simple concepts are the building blocks of apps, games, and websites you use every day!

💡 Remember:

• Stack = Pancakes (top to top)
• Queue = Line (back to front)

Now you’re ready to build amazing things! 🚀