dequeue_ll.c

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



struct node

{
int data;
struct node *prev, *next;

};

struct node *head = NULL, *tail = NULL;

struct node * createNode(int data)

{


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

newnode->data = data;
newnode->next = newnode->prev = NULL;
return (newnode);

}

 

void createSentinels()  {

head = createNode(0);
tail = createNode(0);
head->next = tail;
tail->prev = head;
}



void enqueueAtFront(int data)
{
struct node *newnode, *temp;
newnode = createNode(data);
temp = head->next;
head->next = newnode;
newnode->prev = head;

newnode->next = temp;
temp->prev = newnode;
}

 
void enqueueAtRear(int data)

{
struct node *newnode, *temp;
newnode = createNode(data);

temp = tail->prev;
tail->prev = newnode;
newnode->next = tail;
newnode->prev = temp;
temp->next = newnode;

}

/* deletion at the front of the queue */
void dequeueAtFront()
{

struct node *temp;
if (head->next == tail)

{
printf("Queue is empty\n");
}

else
{
temp = head->next;
head->next = temp->next;
temp->next->prev = head;

free(temp);
}
return;
}


 
/* deletion at the rear of the queue */

void dequeueAtRear()

{
struct node *temp;
if (tail->prev == head)

{
printf("Queue is empty\n");
}
else
{

temp = tail->prev;
tail->prev = temp->prev;
temp->prev->next = tail;
free(temp);
}

return;
}

 
/* display elements present in the queue */

void display()
{
struct node *temp;

if (head->next == tail)
{
printf("Queue is empty\n");
return;
}

temp = head->next;
while (temp != tail)
{

printf("%-3d", temp->data);
temp = temp->next;
}
printf("\n");
}

 

int main()
{
int data, ch;

createSentinels();
while (1)

{
printf("1. Enqueue at front\n2. Enqueue at rear\n");
printf("3. Dequeue at front\n4. Dequeue at rear\n");

printf("5. Display\n6. Exit\n");
printf("Enter your choice:");
scanf("%d", &ch);
switch (ch) 
{

case 1:
printf("Enter the data to insert:");
scanf("%d", &data);
enqueueAtFront(data);
break;

case 2:

printf("Enter ur data to insert:");
scanf("%d", &data);
enqueueAtRear(data);
break;

case 3:
dequeueAtFront();
break;

case 4:

dequeueAtRear();
break;

case 5:
display();
break;

case 6:
exit(0);

default:
printf(" enter correct option\n");
break;
} 
}
return 0;
}