#include <stdio.h>
#include <stdlib.h>
 
// Structure for queue node
struct Node {
    int data;
    struct Node *next;
};
 
// Front and rear pointers
struct Node *front = NULL;
struct Node *rear = NULL;
 
// Function to enqueue (insert) an element into the queue
void enqueue(int value) {
    struct Node *newNode = (struct Node *)malloc(sizeof(struct Node));
    if (newNode == NULL) {
        printf("\nQueue Overflow! Memory not allocated.\n");
        return;
    }
    newNode->data = value;
    newNode->next = NULL;
 
    if (rear == NULL) {
        front = rear = newNode;
    } else {
        rear->next = newNode;
        rear = newNode;
    }
    printf("\n%d enqueued to queue.\n", value);
}
 
// Function to dequeue (remove) an element from the queue
void dequeue() {
    if (front == NULL) {
        printf("\nQueue Underflow! No elements to dequeue.\n");
        return;
    }
 
    struct Node *temp = front;
    printf("\n%d dequeued from queue.\n", front->data);
 
    front = front->next;
 
    // If queue becomes empty
    if (front == NULL)
        rear = NULL;
 
    free(temp);
}
 
// Function to peek (view) the front element
void peek() {
    if (front == NULL)
        printf("\nQueue is empty.\n");
    else
        printf("\nFront element is: %d\n", front->data);
}
 
// Function to display the queue
void display() {
    if (front == NULL) {
        printf("\nQueue is empty.\n");
        return;
    }
 
    struct Node *temp = front;
    printf("\nQueue elements are:\n");
    while (temp != NULL) {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("\n");
}
 
// Main function
int main() {
    int choice, value;
 
    printf("=== QUEUE IMPLEMENTATION USING LINKED LIST ===\n");
 
    while (1) {
        printf("\nMenu:\n");
        printf("1. Enqueue\n");
        printf("2. Dequeue\n");
        printf("3. Peek\n");
        printf("4. Display\n");
        printf("5. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
 
        switch (choice) {
            case 1:
                printf("Enter value to enqueue: ");
                scanf("%d", &value);
                enqueue(value);
                break;
            case 2:
                dequeue();
                break;
            case 3:
                peek();
                break;
            case 4:
                display();
                break;
            case 5:
                printf("\nExiting program.\n");
                exit(0);
            default:
                printf("\nInvalid choice! Try again.\n");
        }
    }
 
    return 0;
}