The ones who are crazy enough to think they can change the world are the ones who do.
- Steve Jobs

# Doubly Linked List

In singly linked list, we can move only in forward direction, whereas in doubly linked list moving forward or backward from any node is possible. The node in the doubly linked list is more complex than the node in the singly linked list because the node in the doubly linked list contains the address of the previous node + its own element + address of the next node.

## Structure - Doubly Linked List

Structure - Doubly Linked List
```struct node
{
struct node *prev;
int data;
struct node *next;
};```

## C program - Doubly Linked List

Here is the program to demonstrate, how to create and use doubly linked list.

```#include <stdio.h>
#include <malloc.h>
struct node
{
struct node *prev;
int data;
struct node *next;
};
struct node *start = NULL;
struct node *create_doublylinkedlist(struct node *);
struct node *display(struct node *);
int main()
{
int choice;
do{
printf("\n1. Create a list");
printf("\n2. Display the list");
printf("\n3. Exit the list");
printf("\n Enter your choice : ");
scanf("%d",&choice);
switch(choice)
{
case 1:
printf("\n Linked List Created Successfully");
break;
case 2:
start = display(start);
break;
}
}while(choice != 3);
return 0;
}
struct node *create_doublylinkedlist(struct node *start)
{
struct node *new_node, *ptr;
int num;
printf("\n Enter -1 to end");
printf("\n Enter the data : ");
scanf("%d",&num);
while(num != -1)
{
new_node = (struct node*)malloc(sizeof(struct node));
new_node -> data = num;
if(start == NULL)
{
start = new_node;
new_node -> next = NULL;
new_node -> prev = NULL;
}
else
{
ptr = start;
while(ptr -> next != NULL)
ptr = ptr ->next;
ptr -> next = new_node;
new_node -> prev = ptr;
new_node -> next = NULL;
}
printf("\n Enter the data: ");
scanf("%d",&num);
}
return start;
}
struct node *display(struct node *start)
{
struct node *ptr;
ptr = start;
while(ptr != NULL)
{
printf("\t %d", ptr -> data);
ptr = ptr -> next;
}
return start;
}
struct node *insert_beg(struct node *start)
{
struct node *new_node;
int num;
printf("\n Enter the data : ");
scanf("%d",&num);
new_node = (struct node *)malloc(sizeof(struct node));
new_node -> data = num;
start -> prev = new_node;
new_node -> next = start;
start -> prev = NULL;
start = new_node;
return start;
}
struct node *insert_end(struct node *start)
{
struct node *ptr, *new_node;
int num;
printf("\n Enter the data : ");
scanf("%d",&num);
new_node = (struct node *)malloc(sizeof(struct node));
new_node -> data = num;
ptr = start;
while(ptr -> next != NULL)
ptr = ptr -> next;
ptr -> next = new_node;
new_node -> prev = ptr;
new_node -> next = NULL;
return start;
}
struct node *insert_before(struct node *start)
{
struct node *new_node, *ptr;
int num, val;
printf("\n Enter the data : ");
scanf("%d",&num);
printf("\n Enter the value before which the data has to be inserted");
scanf("%d",&val);
new_node = (struct node *)malloc(sizeof(struct node));
new_node -> data = num;
ptr = start;
while(ptr -> data != val);
ptr = ptr -> next;
new_node -> next = ptr;
new_node -> prev = ptr -> prev;
ptr -> prev -> next = new_node;
ptr -> prev = new_node;
return start;
}
struct node *insert_after(struct node *start)
{
struct node *new_node, *ptr;
int num, val;
printf("\n Enter the data : ");
scanf("%d",&num);
printf("\n Enter the value after which the data has to be inserted : ");
scanf("%d",&val);
new_node = (struct node *)malloc(sizeof(struct node));
new_node -> data = num;
ptr = start;
while(ptr -> data != val)
ptr = ptr -> next;
new_node -> prev = ptr;
new_node -> next = ptr -> next;
ptr -> next -> prev = new_node;
ptr -> next = new_node;
return start;
}
struct node *delete_beg(struct node *start)
{
struct node *ptr;
ptr = start;
start = start -> next;
start -> prev = NULL;
free(ptr);
return start;
}
struct node *delete_end(struct node *start)
{
struct node *ptr;
ptr = start;
while(ptr -> next != NULL)
ptr = ptr -> next;
ptr -> prev -> next = NULL;
free(ptr);
return start;
}
struct node *delete_after(struct node *start)
{
struct node *ptr, *temp;
int val;
printf("\n Enter the value after which the node has to deleted : ");
scanf("%d",&val);
ptr = start;
while(ptr -> data != val)
ptr = ptr -> next;
temp = ptr -> next;
ptr -> next = temp -> next;
temp -> next -> prev = ptr;
free(temp);
return start;
}
struct node *delete_before(struct node *start)
{
struct node *ptr, *temp;
int val;
printf("\n Enter the value after which the node has to deleted : ");
scanf("%d",&val);
ptr = start;
while(ptr -> data != val)
ptr = ptr -> next;
temp = ptr -> prev;
if(temp == start)
start = delete_beg(start);
else {
ptr -> prev = temp -> prev;
temp -> prev -> next = ptr;
}
free(temp);
return start;
}
struct node *delete_list(struct node *start) {
while(start != NULL)
start = delete_end(start);
free(start);
return start;
}```
```1. Create a list
2. Display the list
3. Exit the list
Enter your choice : 1
Enter -1 to end
Enter the data : 5
Enter the data : 1
Enter the data : 2
Enter the data : -1
Linked List Created Successfully
1. Create a list
2. Display the list
3. Exit the list
Enter your choice : 2
5   1   2
1. Create a list
2. Display the list
3. Exit the list
Enter your choice : 3
```

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