📌 Queues: Operations on Queue

A Queue is a linear data structure that follows the FIFO (First-In, First-Out) principle. This means that the element added first will be removed first, just like a queue of people waiting in line.

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1️⃣ Basic Queue Operations

Operation	Description
Enqueue()	Adds an element to the rear (back) of the queue.
Dequeue()	Removes an element from the front of the queue.
Front() / Peek()	Returns the front element without removing it.
isEmpty()	Checks if the queue is empty.
isFull()	Checks if the queue is full (for fixed-size queues).

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2️⃣ Implementation of Queue in C

(a) Queue Using Arrays

#include <stdio.h>
#define MAX 5  // Maximum queue size

int queue[MAX], front = -1, rear = -1;

// Enqueue operation
void enqueue(int value) {
    if (rear == MAX - 1) {
        printf("Queue Overflow!\n");
        return;
    }
    if (front == -1) front = 0;  // Set front to first element if empty
    queue[++rear] = value;
    printf("Enqueued: %d\n", value);
}

// Dequeue operation
void dequeue() {
    if (front == -1 || front > rear) {
        printf("Queue Underflow!\n");
        return;
    }
    printf("Dequeued: %d\n", queue[front++]);
    if (front > rear) front = rear = -1; // Reset when queue is empty
}

// Display the queue
void display() {
    if (front == -1) {
        printf("Queue is empty!\n");
        return;
    }
    printf("Queue: ");
    for (int i = front; i <= rear; i++)
        printf("%d ", queue[i]);
    printf("\n");
}

int main() {
    enqueue(10);
    enqueue(20);
    enqueue(30);
    display();
    dequeue();
    display();
    return 0;
}

✅ Simple Implementation
❌ Wastes space when elements are dequeued (fixed front).

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(b) Queue Using Linked List

#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node* next;
};

struct Node *front = NULL, *rear = NULL;

// Enqueue operation
void enqueue(int value) {
    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = NULL;
    
    if (rear == NULL) front = rear = newNode;  
    else {
        rear->next = newNode;
        rear = newNode;
    }
    printf("Enqueued: %d\n", value);
}

// Dequeue operation
void dequeue() {
    if (front == NULL) {
        printf("Queue Underflow!\n");
        return;
    }
    struct Node* temp = front;
    printf("Dequeued: %d\n", front->data);
    front = front->next;
    free(temp);

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

// Display queue
void display() {
    if (front == NULL) {
        printf("Queue is empty!\n");
        return;
    }
    struct Node* temp = front;
    printf("Queue: ");
    while (temp != NULL) {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("\n");
}

int main() {
    enqueue(10);
    enqueue(20);
    enqueue(30);
    display();
    dequeue();
    display();
    return 0;
}

✅ Efficient memory usage
✅ No fixed size limit
❌ More complex (pointers needed)

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3️⃣ Special Types of Queues

1️⃣ Circular Queue – Overcomes the issue of wasted space in arrays.
2️⃣ Deque (Double-Ended Queue) – Can add/remove from both front & rear.
3️⃣ Priority Queue – Elements are dequeued based on priority, not order.