C program to implement Queue using single linked list by using global variables

#include <stdio.h>
#include <stdlib.h>
struct node
{
int data;
struct node *next;
};
struct node *f=NULL,*r = NULL,*c;
void enq();
void deq();
void display();
main()
{
          int opt;
while(1)
         {
printf("\n Press 1. Enq\t 2. Deq\t 3. Display \t 4. Exit\n");
               scanf("%d",&opt);
               switch(opt)
             {
                 case 1: enq();
                            break;
                 case 2: deq();
                            break;
                 case 3: display();
                            break;
case 4: exit(0);
            }
        }
}
void enq()
{
int val;
printf("enter value to be enqueued\n");
scanf("%d",&val);
struct node *newnode = (struct node *)malloc(sizeof(struct node));
newnode->data=val;
newnode->next=NULL;
if(f == NULL && r == NULL)
f=r=newnode;
else
{
r->next=newnode;
r= newnode;
}
}

void deq()
{
if(f == NULL && r == NULL)
{
printf("queue is empty");
return;
}
if(f = = r)
       f = r = NULL;
else
{
c=f;
f=f->next;
free(c);
}
}

void display()
{
if(f == NULL && r == NULL)
{
printf("queue is empty");
return;
}
else
{
c=f;
while(c != NULL)
{
printf("%d\t",c->data);
c=c->next;
}
}
}

C program to Implement Queue using linked list

#include <stdio.h>

#include <stdlib.h>
struct Q
{

   int data;

   struct Q *next;

};



void enq(struct Q **,struct Q **);

void deq(struct Q **, struct Q **);

void display(struct Q *,struct Q *);

main()

{

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

     int n,i;

     printf("enter the number of elements");

     scanf("%d",&n);

     for(i=1;i<=n;i++)

          enq(&front,&rear);

     display(front,rear);

     deq(&front,&rear);

     printf("\n After deleting one element\n");

     display(front,rear);

}

void enq(struct Q **front,struct Q **rear)

{

     int val;

     struct Q *newnode =(struct Q *)malloc(sizeof(struct Q));

     printf("enter a value");

     scanf("%d",&val);

     newnode->data=val;

     newnode->next=NULL;

     if(*front == NULL )

         *front = *rear = newnode;

     else

     {

         (*rear)->next=newnode;

         *rear=newnode;

     }

}



void display(struct Q *front,struct Q *rear)

{

        if(front == NULL)

              printf("queue empty");

        else

        {

              while(front != NULL)

              {

                    printf("%d->",front->data);

                    front=front->next;

              }

        }

}

void deq(struct Q **front, struct Q **rear)

{

          if(*front == NULL)

                printf("queue is empty");

          else

         {

                 if(*front == *rear)

                      *front = *rear = NULL;

                 else

                      *front=(*front)->next;

         }

}