### Question

A doubly linked list is a linked data structure that consists of a set of sequentially linked records called nodes. Each node contains two fields, called links, that are references to the previous and to the next node in the sequence of nodes. Write a program to implement following operations in a linked list
1. Append an Element 2. Add at beginning 3. Remove 4. Display

### Source Code

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

/* structure representing a node of the doubly linked list */
struct dnode
{
struct dnode *prev;
int data;
struct dnode *next;
};

struct dnode *start = NULL;

void append(int);
void remov(int);
void display();

int main()
{
int n, ch;
do
{
printf("\n1. Append \n2. Add at beginning \n3. Remove\n4. Display\n0. Exit\n");
printf("\nEnter Choice 0-4? : ");
scanf("%d", &ch);
switch (ch)
{
case 1:
printf("\nEnter number: ");
scanf("%d", &n);
append(n);
break;
case 2:
printf("\nEnter number: ");
scanf("%d", &n);
break;
case 3:
printf("\nEnter number to delete: ");
scanf("%d", &n);
remov(n);
break;
case 4:
display();
break;
}
}while (ch != 0);
}

/* adds a new node at the end of the doubly linked list */
void append(int num)
{
struct dnode *nptr,  *temp = start;

/*create a new node */
nptr = malloc(sizeof(struct dnode));
nptr->data = num;
nptr->next = NULL;
nptr->prev = NULL;

/* if the linked list is empty */
if (start == NULL)
{
start = nptr;
}
else
{
/* traverse the linked list till the last node is reached */
while (temp->next != NULL)
temp = temp->next;

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

/* adds a new node at the begining of the linked list */
{
struct dnode *nptr;

/* create a new node */
nptr = malloc(sizeof(struct dnode));

/* assign data and pointer to the new node */
nptr->prev = NULL;
nptr->data = num;
nptr->next = start;

if (start != NULL)
start->prev = nptr;
start = nptr;
}

/* deletes the specified node from the doubly linked list */
void remov(int num)
{
struct dnode *temp = start;

/* traverse the entire linked list */
while (temp != NULL)
{
/* if node to be deleted is found */
if (temp->data == num)
{
/* if node to be deleted is the first node */
if (temp == start)
{
start = start->next;
start->prev = NULL;
}
else
{
/* if node to be deleted is the last node */
if (temp->next == NULL)
temp->prev->next = NULL;
else
/* if node to be deleted is any intermediate node */
{
temp->prev->next = temp->next;
temp->next->prev = temp->prev;
}
free(temp);
}
return ; /* return back after deletion */
}
temp = temp->next; /* go to next node */
}
}

/* displays the contents of the linked list */
void display()
{
struct dnode *temp = start;
printf("\n");

/* traverse the entire linked list */
while (temp != NULL)
{
printf("%d\t", temp->data);
temp = temp->next;
}
}``````