#include<stdio.h>   

#include<stdlib.h>  

struct node   

{  

    int data;   

    struct node *next;  

};  

struct node *front;  

struct node *rear;   

void insert();  

void delete();  

void display();  

void main ()  

{  

    int choice;   

    while(choice != 4)   

    {     

        printf("\n*************************Main Menu*****************************\n");  

        printf("\n=================================================================\n");  

        printf("\n1.insert an element\n2.Delete an element\n3.Display the queue\n4.Exit\n");  

        printf("\nEnter your choice ?");  

        scanf("%d",& choice);  

        switch(choice)  

        {  

            case 1:  

            insert();  

            break;  

            case 2:  

            delete();  

            break;  

            case 3:  

            display();  

            break;  

            case 4:  

            exit(0);  

            break;  

            default:   

            printf("\nEnter valid choice??\n");  

        }  

    }  

}  

void insert()  

{  

    struct node *ptr;  

    int item;   

      

    ptr = (struct node *) malloc (sizeof(struct node));  

    if(ptr == NULL)  

    {  

        printf("\nOVERFLOW\n");  

        return;  

    }  

    else  

    {   

        printf("\nEnter value?\n");  

        scanf("%d",&item);  

        ptr -> data = item;  

        if(front == NULL)  

        {  

            front = ptr;  

            rear = ptr;   

            front -> next = NULL;  

            rear -> next = NULL;  

        }  

        else   

        {  

            rear -> next = ptr;  

            rear = ptr;  

            rear->next = NULL;  

        }  

    }  

}     

void delete ()  

{  

    struct node *ptr;  

    if(front == NULL)  

    {  

        printf("\nUNDERFLOW\n");  

        return;  

    }  

    else   

    {  

        ptr = front;  

        front = front -> next;  

        free(ptr);  

    }  

}  

void display()  

{  

    struct node *ptr;  

    ptr = front;      

    if(front == NULL)  

    {  

        printf("\nEmpty queue\n");  

    }  

    else  

    {   printf("\nprinting values .....\n");  

        while(ptr != NULL)   

        {  

            printf("\n%d\n",ptr -> data);  

            ptr = ptr -> next;  

        }  

    }  

}