Topic 2

Queue (Array & LL)

FIFO — First In, First Out

A queue works like a real-life waiting line. First person to join the line gets served first.

Operations: enqueue() — add at the rear, dequeue() — remove from the front

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Array Queue: Uses front and rear pointers (indexes). Starts at -1.

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LL Queue: Uses front and rear node pointers. Starts at NULL.

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Enqueue: Array arr[++rear] | LL inserts Node at rear.

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Dequeue: Array front++ | LL removes Node at front.

Array Queue vs Linked List Queue

Aspect	Array Queue	Linked List Queue
Size	Fixed	Dynamic
Pointers	int front = -1, rear = -1	Node front = NULL, rear = NULL
Enqueue	arr[++rear] = value	rear->next = newNode; rear = newNode;
Dequeue	front++	Node* temp = front; front = front->next; delete temp;
Queue Full	rear == size - 1	Never full (memory permitting)

#include <iostream>
using namespace std;

class Queue {
private:
    int front, rear, size;
    int* arr;

public:
    Queue(int s) {
        front = rear = -1;
        size = s;
        arr = new int[size];
    }

    bool isFull() {
        return rear == size - 1;
    }

    bool isEmpty() {
        return front == -1 || front > rear;
    }

    // Enqueue
    void enqueue(int value) {
        if (isFull()) {
            cout << "Queue is Full!" << endl;
            return;
        }
        if (front == -1) front = 0;
        arr[++rear] = value;
        cout << value << " enqueued to queue" << endl;
    }

    // Dequeue
    void dequeue() {
        if (isEmpty()) {
            cout << "Queue is Empty!" << endl;
            return;
        }
        cout << arr[front] << " dequeued from queue" << endl;
        front++;
    }

    // Display
    void display() {
        if (isEmpty()) {
            cout << "Queue is Empty!" << endl;
            return;
        }
        cout << "Queue elements: ";
        for (int i = front; i <= rear; i++) {
            cout << arr[i] << " ";
        }
        cout << endl;
    }

    ~Queue() {
        delete[] arr;
    }
};

int main() {
    Queue q(5);
    q.enqueue(10);
    q.enqueue(20);
    q.enqueue(30);
    q.display();        // 10 20 30

    q.dequeue();
    q.display();        // 20 30

    return 0;
}

#include <iostream>
using namespace std;

class Node {
public:
    int data;
    Node* next;
};

class Queue {
private:
    Node *front, *rear;
public:
    Queue() {
        front = rear = NULL;
    }

    bool isEmpty() {
        return front == NULL;
    }

    void enqueue(int value) {
        Node* newNode = new Node();
        newNode->data = value;
        newNode->next = NULL;

        if (rear == NULL) {
            front = rear = newNode;
            cout << value << " enqueued to queue" << endl;
            return;
        }

        rear->next = newNode;
        rear = newNode;
        cout << value << " enqueued to queue" << endl;
    }

    void dequeue() {
        if (isEmpty()) {
            cout << "Queue is Empty!" << endl;
            return;
        }

        Node* temp = front;
        int dequeuedValue = temp->data;
        front = front->next;

        if (front == NULL) {
            rear = NULL;
        }

        cout << dequeuedValue << " dequeued from queue" << endl;
        delete temp;
    }

    void display() {
        if (isEmpty()) {
            cout << "Queue is Empty!" << endl;
            return;
        }
        cout << "Queue elements: ";
        Node* current = front;
        while (current != NULL) {
            cout << current->data << " ";
            current = current->next;
        }
        cout << endl;
    }

    ~Queue() {
        while (!isEmpty()) {
            Node* temp = front;
            front = front->next;
            delete temp;
        }
        rear = NULL;
    }
};

int main() {
    Queue q;
    q.enqueue(10);
    q.enqueue(20);
    q.enqueue(30);
    q.display();

    q.dequeue();
    q.display();

    return 0;
}