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


typedef int element;
typedef struct queue *queueptr;


typedef struct queue {
    element item;
    queueptr link;
} queue;


queueptr front = NULL;
queueptr rear = NULL;

int ISEMPTY();
void addq(element item);
element deleteq();
void display();


int main() {
    int choice, item;

    while (1) {
        printf("\n--- Single Queue Menu ---\n");
        printf("1. Add to Queue\n");
        printf("2. Delete from Queue\n");
        printf("3. Display Queue\n");
        printf("4. Exit\n");
        printf("Enter your choice: ");

        scanf("%d", &choice);


        switch (choice) {
            case 1:
                printf("Enter item to add: ");
                scanf("%d", &item);
                addq(item);
                printf("Item %d added to queue.\n", item);
                break;

            case 2:

                item = deleteq();
                printf("Deleted item %d from queue.\n", item);
                break;

            case 3:
                display();
                break;

            case 4:
                printf("Exiting program.\n");
                exit(0);

            default:
                printf("Invalid choice. Please try again.\n");
        }
    }

    return 0;
}

int ISEMPTY() {
    return (front == NULL);
}


void addq(element item) {

    queueptr temp = (queueptr)malloc(sizeof(queue));


    if (temp == NULL) {
        fprintf(stderr, "Memory allocation failed. The queue is full!\n");
        exit(1);
    }

    temp->item = item;
    temp->link = NULL;

    if (ISEMPTY()) {
        front = temp;
    } else {
        rear->link = temp;
    }
    rear = temp;
}


element deleteq() {
    queueptr temp = front;
    element item;

    if (ISEMPTY()) {
        fprintf(stderr, "The queue is empty, cannot delete.\n");
        exit(1);
    }

    item = temp->item;
    front = temp->link;
    free(temp);
    if (ISEMPTY()) {
        rear = NULL;
    }
    return item;
}


void display() {
    queueptr temp = front;

    if (ISEMPTY()) {
        printf("Queue is empty.\n");
        return;
    }

    printf("Queue contents (front to rear): ");
    while (temp != NULL) {
        printf("%d ", temp->item);
        temp = temp->link;
    }
    printf("\n");
}