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How many pointers are there for each node in doubly linked list?

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Doubly Linked In C++

As in the singly linked list, the doubly linked list also has a head and a tail. The previous pointer of the head is set to NULL as this is the first node. The next pointer of the tail node is set to NULL as this is the last node.

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A basic layout of the doubly linked list is shown in the below diagram.

How many pointers are there for each node in doubly linked list?

Following is representation of a DLL node in C language.




/* Node of a doubly linked list */
class Node
{
public:
int data;
Node* next; // Pointer to next node in DLL
Node* prev; // Pointer to previous node in DLL
};
// This code is contributed by shivanisinghss2110




/* Node of a doubly linked list */
struct Node {
int data;
struct Node* next; // Pointer to next node in DLL
struct Node* prev; // Pointer to previous node in DLL
};




// Class for Doubly Linked List
public class DLL {
Node head; // head of list
/* Doubly Linked list Node*/
class Node {
int data;
Node prev;
Node next;
// Constructor to create a new node
// next and prev is by default initialized as null
Node(int d) { data = d; }
}
}




# Node of a doubly linked list
class Node:
def __init__(self, next=None, prev=None, data=None):
self.next = next # reference to next node in DLL
self.prev = prev # reference to previous node in DLL
self.data = data




// Class for Doubly Linked List
public class DLL {
Node head; // head of list
/* Doubly Linked list Node*/
public class Node {
public int data;
public Node prev;
public Node next;
// Constructor to create a new node
// next and prev is by default initialized as null
Node(int d) { data = d; }
}
}
// This code contributed by gauravrajput1




Following are advantages/disadvantages of doubly linked list over singly linked list.
Advantages over singly linked list
1) A DLL can be traversed in both forward and backward direction.
2) The delete operation in DLL is more efficient if pointer to the node to be deleted is given.
3) We can quickly insert a new node before a given node.
In singly linked list, to delete a node, pointer to the previous node is needed. To get this previous node, sometimes the list is traversed. In DLL, we can get the previous node using previous pointer.

Disadvantages over singly linked list
1) Every node of DLL Require extra space for an previous pointer. It is possible to implement DLL with single pointer though (See this and this).
2) All operations require an extra pointer previous to be maintained. For example, in insertion, we need to modify previous pointers together with next pointers. For example in following functions for insertions at different positions, we need 1 or 2 extra steps to set previous pointer.
Insertion
A node can be added in four ways
1) At the front of the DLL
2) After a given node.
3) At the end of the DLL
4) Before a given node.



Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

1) Add a node at the front: (A 5 steps process)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of DLL. For example if the given Linked List is 10152025 and we add an item 5 at the front, then the Linked List becomes 510152025. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)

How many pointers are there for each node in doubly linked list?

Following are the 5 steps to add node at the front.




/* Given a reference (pointer to pointer)
to the head of a list
and an int, inserts a new node on the
front of the list. */
void push(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head
and previous as NULL */
new_node->next = (*head_ref);
new_node->prev = NULL;
/* 4. change prev of head node to new node */
if ((*head_ref) != NULL)
(*head_ref)->prev = new_node;
/* 5. move the head to point to the new node */
(*head_ref) = new_node;
}
// This code is contributed by shivanisinghss2110




/* Given a reference (pointer to pointer) to the head of a list
and an int, inserts a new node on the front of the list. */
void push(struct Node** head_ref, int new_data)
{
/* 1. allocate node */
struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head and previous as NULL */
new_node->next = (*head_ref);
new_node->prev = NULL;
/* 4. change prev of head node to new node */
if ((*head_ref) != NULL)
(*head_ref)->prev = new_node;
/* 5. move the head to point to the new node */
(*head_ref) = new_node;
}




// Adding a node at the front of the list
public void push(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_Node = new Node(new_data);
/* 3. Make next of new node as head and previous as NULL */
new_Node.next = head;
new_Node.prev = null;
/* 4. change prev of head node to new node */
if (head != null)
head.prev = new_Node;
/* 5. move the head to point to the new node */
head = new_Node;
}




# Adding a node at the front of the list
def push(self, new_data):
# 1 & 2: Allocate the Node & Put in the data
new_node = Node(data = new_data)
# 3. Make next of new node as head and previous as NULL
new_node.next = self.head
new_node.prev = None
# 4. change prev of head node to new node
if self.head is not None:
self.head.prev = new_node
# 5. move the head to point to the new node
self.head = new_node
# This code is contributed by jatinreaper




// Adding a node at the front of the list
public void push(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_Node = new Node(new_data);
/* 3. Make next of new node as head and previous as NULL */
new_Node.next = head;
new_Node.prev = null;
/* 4. change prev of head node to new node */
if (head != null)
head.prev = new_Node;
/* 5. move the head to point to the new node */
head = new_Node;
}
// This code is contributed by aashish2995




// Adding a node at the front of the list
function push(new_data)
{
/* 1. allocate node
* 2. put in the data */
let new_Node = new Node(new_data);
/* 3. Make next of new node as head and previous as NULL */
new_Node.next = head;
new_Node.prev = null;
/* 4. change prev of head node to new node */
if (head != null)
head.prev = new_Node;
/* 5. move the head to point to the new node */
head = new_Node;
}
// This code is contributed by saurabh_jaiswal.

Four steps of the above five steps are same as the 4 steps used for inserting at the front in singly linked list. The only extra step is to change previous of head.
2) Add a node after a given node.: (A 7 steps process)
We are given pointer to a node as prev_node, and the new node is inserted after the given node.

How many pointers are there for each node in doubly linked list?




/* Given a node as prev_node, insert
a new node after the given node */
void insertAfter(Node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL)
{
cout<<"the given previous node cannot be NULL";
return;
}
/* 2. allocate new node */
Node* new_node = new Node();
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. Make the next of prev_node as new_node */
prev_node->next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node->prev = prev_node;
/* 7. Change previous of new_node's next node */
if (new_node->next != NULL)
new_node->next->prev = new_node;
}
// This code is contributed by shivanisinghss2110.




/* Given a node as prev_node, insert a new node after the given node */
void insertAfter(struct Node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL) {
printf("the given previous node cannot be NULL");
return;
}
/* 2. allocate new node */
struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. Make the next of prev_node as new_node */
prev_node->next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node->prev = prev_node;
/* 7. Change previous of new_node's next node */
if (new_node->next != NULL)
new_node->next->prev = new_node;
}




/* Given a node as prev_node, insert a new node after the given node */
public void InsertAfter(Node prev_Node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_Node == null) {
System.out.println("The given previous node cannot be NULL ");
return;
}
/* 2. allocate node
* 3. put in the data */
Node new_node = new Node(new_data);
/* 4. Make next of new node as next of prev_node */
new_node.next = prev_Node.next;
/* 5. Make the next of prev_node as new_node */
prev_Node.next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node.prev = prev_Node;
/* 7. Change previous of new_node's next node */
if (new_node.next != null)
new_node.next.prev = new_node;
}




# Given a node as prev_node, insert
# a new node after the given node
def insertAfter(self, prev_node, new_data):
# 1. check if the given prev_node is NULL
if prev_node is None:
print("This node doesn't exist in DLL")
return
#2. allocate node & 3. put in the data
new_node = Node(data = new_data)
# 4. Make next of new node as next of prev_node
new_node.next = prev_node.next
# 5. Make the next of prev_node as new_node
prev_node.next = new_node
# 6. Make prev_node as previous of new_node
new_node.prev = prev_node
# 7. Change previous of new_node's next node */
if new_node.next is not None:
new_node.next.prev = new_node
# This code is contributed by jatinreaper




/* Given a node as prev_node, insert a new node after the given node */
public void InsertAfter(Node prev_Node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_Node == null) {
Console.WriteLine("The given previous node cannot be NULL ");
return;
}
/* 2. allocate node
* 3. put in the data */
Node new_node = new Node(new_data);
/* 4. Make next of new node as next of prev_node */
new_node.next = prev_Node.next;
/* 5. Make the next of prev_node as new_node */
prev_Node.next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node.prev = prev_Node;
/* 7. Change previous of new_node's next node */
if (new_node.next != null)
new_node.next.prev = new_node;
}
// This code is contributed by aashish2995




Five of the above steps step process are same as the 5 steps used for inserting after a given node in singly linked list. The two extra steps are needed to change previous pointer of new node and previous pointer of new node’s next node.
3) Add a node at the end: (7 steps process)
The new node is always added after the last node of the given Linked List. For example if the given DLL is 510152025 and we add an item 30 at the end, then the DLL becomes 51015202530.
Since a Linked List is typically represented by the head of it, we have to traverse the list till end and then change the next of last node to new node.

How many pointers are there for each node in doubly linked list?

Following are the 7 steps to add node at the end.




/* Given a reference (pointer to pointer) to the head
of a DLL and an int, appends a new node at the end */
void append(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
Node* last = *head_ref; /* used in step 5*/
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be the last node, so
make next of it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty, then make the new
node as head */
if (*head_ref == NULL)
{
new_node->prev = NULL;
*head_ref = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
last = last->next;
/* 6. Change the next of last node */
last->next = new_node;
/* 7. Make last node as previous of new node */
new_node->prev = last;
return;
}
// This code is contributed by shivanisinghss2110




/* Given a reference (pointer to pointer) to the head
of a DLL and an int, appends a new node at the end */
void append(struct Node** head_ref, int new_data)
{
/* 1. allocate node */
struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
struct Node* last = *head_ref; /* used in step 5*/
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be the last node, so
make next of it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty, then make the new
node as head */
if (*head_ref == NULL) {
new_node->prev = NULL;
*head_ref = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
last = last->next;
/* 6. Change the next of last node */
last->next = new_node;
/* 7. Make last node as previous of new node */
new_node->prev = last;
return;
}




// Add a node at the end of the list
void append(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_node = new Node(new_data);
Node last = head; /* used in step 5*/
/* 3. This new node is going to be the last node, so
* make next of it as NULL*/
new_node.next = null;
/* 4. If the Linked List is empty, then make the new
* node as head */
if (head == null) {
new_node.prev = null;
head = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
/* 7. Make last node as previous of new node */
new_node.prev = last;
}




# Add a node at the end of the DLL
def append(self, new_data):
# 1. allocate node 2. put in the data
new_node = Node(data = new_data)
last = self.head
# 3. This new node is going to be the
# last node, so make next of it as NULL
new_node.next = None
# 4. If the Linked List is empty, then
# make the new node as head
if self.head is None:
new_node.prev = None
self.head = new_node
return
# 5. Else traverse till the last node
while (last.next is not None):
last = last.next
# 6. Change the next of last node
last.next = new_node
# 7. Make last node as previous of new node */
new_node.prev = last
# This code is contributed by jatinreaper




// Add a node at the end of the list
void append(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_node = new Node(new_data);
Node last = head; /* used in step 5*/
/* 3. This new node is going
to be the last node, so
* make next of it as NULL*/
new_node.next = null;
/* 4. If the Linked List is empty,
then make the new * node as head */
if (head == null)
{
new_node.prev = null;
head = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
/* 7. Make last node as previous of new node */
new_node.prev = last;
}
// This code is contributed by shivanisinghss2110




Six of the above 7 steps are same as the 6 steps used for inserting after a given node in singly linked list. The one extra step is needed to change previous pointer of new node.
4) Add a node before a given node:

Steps
Let the pointer to this given node be next_node and the data of the new node to be added as new_data.

  1. Check if the next_node is NULL or not. If it’s NULL, return from the function because any new node can not be added before a NULL
  2. Allocate memory for the new node, let it be called new_node
  3. Set new_node->data = new_data
  4. Set the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prev
  5. Set the previous pointer of the next_node as the new_node, next_node->prev = new_node
  6. Set the next pointer of this new_node as the next_node, new_node->next = next_node;
  7. If the previous node of the new_node is not NULL, then set the next pointer of this previous node as new_node, new_node->prev->next = new_node
  8. Else, if the prev of new_node is NULL, it will be the new head node. So, make (*head_ref) = new_node.

How many pointers are there for each node in doubly linked list?

Below is the implementation of the above approach:

Code block

Output:

Created DLL is:

Traversal in forward Direction

9 1 5 7 6

Traversal in reverse direction

6 7 5 1 9

A complete working program to test above functions.
Following is complete program to test above functions.




// A complete working C++ program to
// demonstrate all insertion methods
#include
using namespace std;
// A linked list node
class Node
{
public:
int data;
Node* next;
Node* prev;
};
/* Given a reference (pointer to pointer)
to the head of a list
and an int, inserts a new node on the
front of the list. */
void push(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head
and previous as NULL */
new_node->next = (*head_ref);
new_node->prev = NULL;
/* 4. change prev of head node to new node */
if ((*head_ref) != NULL)
(*head_ref)->prev = new_node;
/* 5. move the head to point to the new node */
(*head_ref) = new_node;
}
/* Given a node as prev_node, insert
a new node after the given node */
void insertAfter(Node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL)
{
cout<<"the given previous node cannot be NULL";
return;
}
/* 2. allocate new node */
Node* new_node = new Node();
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. Make the next of prev_node as new_node */
prev_node->next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node->prev = prev_node;
/* 7. Change previous of new_node's next node */
if (new_node->next != NULL)
new_node->next->prev = new_node;
}
/* Given a reference (pointer to pointer) to the head
of a DLL and an int, appends a new node at the end */
void append(Node** head_ref, int new_data)
{
/* 1. allocate node */
Node* new_node = new Node();
Node* last = *head_ref; /* used in step 5*/
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be the last node, so
make next of it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty, then make the new
node as head */
if (*head_ref == NULL)
{
new_node->prev = NULL;
*head_ref = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
last = last->next;
/* 6. Change the next of last node */
last->next = new_node;
/* 7. Make last node as previous of new node */
new_node->prev = last;
return;
}
// This function prints contents of
// linked list starting from the given node
void printList(Node* node)
{
Node* last;
cout<<"\nTraversal in forward direction \n";
while (node != NULL)
{
cout<<" "<data<<" ";
last = node;
node = node->next;
}
cout<<"\nTraversal in reverse direction \n";
while (last != NULL)
{
cout<<" "<data<<" ";
last = last->prev;
}
}
/* Driver program to test above functions*/
int main()
{
/* Start with the empty list */
Node* head = NULL;
// Insert 6. So linked list becomes 6->NULL
append(&head, 6);
// Insert 7 at the beginning. So
// linked list becomes 7->6->NULL
push(&head, 7);
// Insert 1 at the beginning. So
// linked list becomes 1->7->6->NULL
push(&head, 1);
// Insert 4 at the end. So linked
// list becomes 1->7->6->4->NULL
append(&head, 4);
// Insert 8, after 7. So linked
// list becomes 1->7->8->6->4->NULL
insertAfter(head->next, 8);
cout << "Created DLL is: ";
printList(head);
return 0;
}
// This is code is contributed by rathbhupendra




// A complete working C program to
// demonstrate all insertion
// methods
#include
#include
// A linked list node
struct Node {
int data;
struct Node* next;
struct Node* prev;
};
/* Given a reference (pointer to pointer) to the head of a
list and an int, inserts a new node on the front of the
list. */
void push(struct Node** head_ref, int new_data)
{
/* 1. allocate node */
struct Node* new_node
= (struct Node*)malloc(sizeof(struct Node));
/* 2. put in the data */
new_node->data = new_data;
/* 3. Make next of new node as head and previous as NULL
*/
new_node->next = (*head_ref);
new_node->prev = NULL;
/* 4. change prev of head node to new node */
if ((*head_ref) != NULL)
(*head_ref)->prev = new_node;
/* 5. move the head to point to the new node */
(*head_ref) = new_node;
}
/* Given a node as prev_node, insert a new node after the
* given node */
void insertAfter(struct Node* prev_node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_node == NULL) {
printf("the given previous node cannot be NULL");
return;
}
/* 2. allocate new node */
struct Node* new_node
= (struct Node*)malloc(sizeof(struct Node));
/* 3. put in the data */
new_node->data = new_data;
/* 4. Make next of new node as next of prev_node */
new_node->next = prev_node->next;
/* 5. Make the next of prev_node as new_node */
prev_node->next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node->prev = prev_node;
/* 7. Change previous of new_node's next node */
if (new_node->next != NULL)
new_node->next->prev = new_node;
}
/* Given a reference (pointer to pointer) to the head
of a DLL and an int, appends a new node at the end */
void append(struct Node** head_ref, int new_data)
{
/* 1. allocate node */
struct Node* new_node
= (struct Node*)malloc(sizeof(struct Node));
struct Node* last = *head_ref; /* used in step 5*/
/* 2. put in the data */
new_node->data = new_data;
/* 3. This new node is going to be the last node, so
make next of it as NULL*/
new_node->next = NULL;
/* 4. If the Linked List is empty, then make the new
node as head */
if (*head_ref == NULL) {
new_node->prev = NULL;
*head_ref = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last->next != NULL)
last = last->next;
/* 6. Change the next of last node */
last->next = new_node;
/* 7. Make last node as previous of new node */
new_node->prev = last;
return;
}
// This function prints contents of linked list starting
// from the given node
void printList(struct Node* node)
{
struct Node* last;
printf("\nTraversal in forward direction \n");
while (node != NULL) {
printf(" %d ", node->data);
last = node;
node = node->next;
}
printf("\nTraversal in reverse direction \n");
while (last != NULL) {
printf(" %d ", last->data);
last = last->prev;
}
}
/* Driver program to test above functions*/
int main()
{
/* Start with the empty list */
struct Node* head = NULL;
// Insert 6. So linked list becomes 6->NULL
append(&head, 6);
// Insert 7 at the beginning. So linked list becomes
// 7->6->NULL
push(&head, 7);
// Insert 1 at the beginning. So linked list becomes
// 1->7->6->NULL
push(&head, 1);
// Insert 4 at the end. So linked list becomes
// 1->7->6->4->NULL
append(&head, 4);
// Insert 8, after 7. So linked list becomes
// 1->7->8->6->4->NULL
insertAfter(head->next, 8);
printf("Created DLL is: ");
printList(head);
getchar();
return 0;
}




// A complete working Java program to demonstrate all
// Class for Doubly Linked List
public class DLL {
Node head; // head of list
/* Doubly Linked list Node*/
class Node {
int data;
Node prev;
Node next;
// Constructor to create a new node
// next and prev is by default initialized as null
Node(int d) { data = d; }
}
// Adding a node at the front of the list
public void push(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_Node = new Node(new_data);
/* 3. Make next of new node as head and previous as NULL */
new_Node.next = head;
new_Node.prev = null;
/* 4. change prev of head node to new node */
if (head != null)
head.prev = new_Node;
/* 5. move the head to point to the new node */
head = new_Node;
}
// Add a node before the given node
public void InsertBefore(Node next_node, int new_data)
{
/*Check if the given nx_node is NULL*/
if(next_node == null)
{
System.out.println("The given next node can not be NULL");
return;
}
//Allocate node, put in the data
Node new_node = new Node(new_data);
//Making prev of new node as prev of next node
new_node.prev = next_node.prev;
//Making prev of next node as new node
next_node.prev = new_node;
//Making next of new node as next node
new_node.next = next_node;
//Check if new node is added as head
if(new_node.prev != null)
new_node.prev.next = new_node;
else
head = new_node;
}
/* Given a node as prev_node, insert
a new node after the given node */
public void InsertAfter(Node prev_Node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_Node == null) {
System.out.println("The given previous node cannot be NULL ");
return;
}
/* 2. allocate node
* 3. put in the data */
Node new_node = new Node(new_data);
/* 4. Make next of new node as next of prev_node */
new_node.next = prev_Node.next;
/* 5. Make the next of prev_node as new_node */
prev_Node.next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node.prev = prev_Node;
/* 7. Change previous of new_node's next node */
if (new_node.next != null)
new_node.next.prev = new_node;
}
// Add a node at the end of the list
void append(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_node = new Node(new_data);
Node last = head; /* used in step 5*/
/* 3. This new node is going to be the last node, so
* make next of it as NULL*/
new_node.next = null;
/* 4. If the Linked List is empty, then make the new
* node as head */
if (head == null) {
new_node.prev = null;
head = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
/* 7. Make last node as previous of new node */
new_node.prev = last;
}
// This function prints contents of
// linked list starting from the given node
public void printlist(Node node)
{
Node last = null;
System.out.println("Traversal in forward Direction");
while (node != null) {
System.out.print(node.data + " ");
last = node;
node = node.next;
}
System.out.println();
System.out.println("Traversal in reverse direction");
while (last != null) {
System.out.print(last.data + " ");
last = last.prev;
}
}
/* Driver program to test above functions*/
public static void main(String[] args)
{
/* Start with the empty list */
DLL dll = new DLL();
// Insert 6. So linked list becomes 6->NULL
dll.append(6);
// Insert 7 at the beginning. So
// linked list becomes 7->6->NULL
dll.push(7);
// Insert 1 at the beginning. So
// linked list becomes 1->7->6->NULL
dll.push(1);
// Insert 4 at the end. So linked
// list becomes 1->7->6->4->NULL
dll.append(4);
// Insert 8, after 7. So linked
// list becomes 1->7->8->6->4->NULL
dll.InsertAfter(dll.head.next, 8);
// Insert 5, before 8.So linked
// list becomes 1->7->5->8->6->4
dll.InsertBefore(dll.head.next.next, 5);
System.out.println("Created DLL is: ");
dll.printlist(dll.head);
}
}
// This code is contributed by Sumit Ghosh




# A complete working Python
# program to demonstrate all
# insertion methods
# A linked list node
class Node:
# Constructor to create a new node
def __init__(self, data):
self.data = data
self.next = None
self.prev = None
# Class to create a Doubly Linked List
class DoublyLinkedList:
# Constructor for empty Doubly Linked List
def __init__(self):
self.head = None
# Given a reference to the head of a list and an
# integer, inserts a new node on the front of list
def push(self, new_data):
# 1. Allocates node
# 2. Put the data in it
new_node = Node(new_data)
# 3. Make next of new node as head and
# previous as None (already None)
new_node.next = self.head
# 4. change prev of head node to new_node
if self.head is not None:
self.head.prev = new_node
# 5. move the head to point to the new node
self.head = new_node
# Given a node as prev_node, insert a new node after
# the given node
def insertAfter(self, prev_node, new_data):
# 1. Check if the given prev_node is None
if prev_node is None:
print("the given previous node cannot be NULL")
return
# 2. allocate new node
# 3. put in the data
new_node = Node(new_data)
# 4. Make net of new node as next of prev node
new_node.next = prev_node.next
# 5. Make prev_node as previous of new_node
prev_node.next = new_node
# 6. Make prev_node ass previous of new_node
new_node.prev = prev_node
# 7. Change previous of new_nodes's next node
if new_node.next:
new_node.next.prev = new_node
# Given a reference to the head of DLL and integer,
# appends a new node at the end
def append(self, new_data):
# 1. Allocates node
# 2. Put in the data
new_node = Node(new_data)
# 3. This new node is going to be the last node,
# so make next of it as None
# (It already is initialized as None)
# 4. If the Linked List is empty, then make the
# new node as head
if self.head is None:
self.head = new_node
return
# 5. Else traverse till the last node
last = self.head
while last.next:
last = last.next
# 6. Change the next of last node
last.next = new_node
# 7. Make last node as previous of new node
new_node.prev = last
return
# This function prints contents of linked list
# starting from the given node
def printList(self, node):
print("\nTraversal in forward direction")
while node:
print(" {}".format(node.data))
last = node
node = node.next
print("\nTraversal in reverse direction")
while last:
print(" {}".format(last.data))
last = last.prev
# Driver program to test above functions
# Start with empty list
llist = DoublyLinkedList()
# Insert 6. So the list becomes 6->None
llist.append(6)
# Insert 7 at the beginning.
# So linked list becomes 7->6->None
llist.push(7)
# Insert 1 at the beginning.
# So linked list becomes 1->7->6->None
llist.push(1)
# Insert 4 at the end.
# So linked list becomes 1->7->6->4->None
llist.append(4)
# Insert 8, after 7.
# So linked list becomes 1->7->8->6->4->None
llist.insertAfter(llist.head.next, 8)
print ("Created DLL is: ")
llist.printList(llist.head)
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)




// A complete working C# program to demonstrate all
using System;
// Class for Doubly Linked List
public class DLL
{
Node head; // head of list
/* Doubly Linked list Node*/
public class Node
{
public int data;
public Node prev;
public Node next;
// Constructor to create a new node
// next and prev is by default initialized as null
public Node(int d)
{
data = d;
}
}
// Adding a node at the front of the list
public void push(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_Node = new Node(new_data);
/* 3. Make next of new node as
head and previous as NULL */
new_Node.next = head;
new_Node.prev = null;
/* 4. change prev of head node to new node */
if (head != null)
head.prev = new_Node;
/* 5. move the head to point to the new node */
head = new_Node;
}
/* Given a node as prev_node, insert
a new node after the given node */
public void InsertAfter(Node prev_Node, int new_data)
{
/*1. check if the given prev_node is NULL */
if (prev_Node == null)
{
Console.WriteLine("The given previous node cannot be NULL ");
return;
}
/* 2. allocate node
* 3. put in the data */
Node new_node = new Node(new_data);
/* 4. Make next of new node as next of prev_node */
new_node.next = prev_Node.next;
/* 5. Make the next of prev_node as new_node */
prev_Node.next = new_node;
/* 6. Make prev_node as previous of new_node */
new_node.prev = prev_Node;
/* 7. Change previous of new_node's next node */
if (new_node.next != null)
new_node.next.prev = new_node;
}
// Add a node at the end of the list
void append(int new_data)
{
/* 1. allocate node
* 2. put in the data */
Node new_node = new Node(new_data);
Node last = head; /* used in step 5*/
/* 3. This new node is going
to be the last node, so
* make next of it as NULL*/
new_node.next = null;
/* 4. If the Linked List is empty,
then make the new * node as head */
if (head == null)
{
new_node.prev = null;
head = new_node;
return;
}
/* 5. Else traverse till the last node */
while (last.next != null)
last = last.next;
/* 6. Change the next of last node */
last.next = new_node;
/* 7. Make last node as previous of new node */
new_node.prev = last;
}
// This function prints contents of
// linked list starting from the given node
public void printlist(Node node)
{
Node last = null;
Console.WriteLine("Traversal in forward Direction");
while (node != null) {
Console.Write(node.data + " ");
last = node;
node = node.next;
}
Console.WriteLine();
Console.WriteLine("Traversal in reverse direction");
while (last != null) {
Console.Write(last.data + " ");
last = last.prev;
}
}
/* Driver code*/
public static void Main(String[] args)
{
/* Start with the empty list */
DLL dll = new DLL();
// Insert 6. So linked list becomes 6->NULL
dll.append(6);
// Insert 7 at the beginning.
// So linked list becomes 7->6->NULL
dll.push(7);
// Insert 1 at the beginning.
// So linked list becomes 1->7->6->NULL
dll.push(1);
// Insert 4 at the end. So linked list
// becomes 1->7->6->4->NULL
dll.append(4);
// Insert 8, after 7. So linked list
// becomes 1->7->8->6->4->NULL
dll.InsertAfter(dll.head.next, 8);
Console.WriteLine("Created DLL is: ");
dll.printlist(dll.head);
}
}
// This code is contributed by 29AjayKumar




Output:

Created DLL is: Traversal in forward Direction 1 7 5 8 6 4 Traversal in reverse direction 4 6 8 5 7 1

Also see: Delete a node in double Link List
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

How many pointers are there for each node in doubly linked list?




Article Tags :
Data Structures
Linked List
doubly linked list
Visa
Practice Tags :
Visa
Data Structures
Linked List
Read Full Article

Doubly linked list

Doubly linked list is a complex type of linked list in which a node contains a pointer to the previous as well as the next node in the sequence. Therefore, in a doubly linked list, a node consists of three parts: node data, pointer to the next node in sequence (next pointer) , pointer to the previous node (previous pointer). A sample node in a doubly linked list is shown in the figure.


How many pointers are there for each node in doubly linked list?

A doubly linked list containing three nodes having numbers from 1 to 3 in their data part, is shown in the following image.


How many pointers are there for each node in doubly linked list?

In C, structure of a node in doubly linked list can be given as :

The prev part of the first node and the next part of the last node will always contain null indicating end in each direction.

In a singly linked list, we could traverse only in one direction, because each node contains address of the next node and it doesn't have any record of its previous nodes. However, doubly linked list overcome this limitation of singly linked list. Due to the fact that, each node of the list contains the address of its previous node, we can find all the details about the previous node as well by using the previous address stored inside the previous part of each node.

Doubly Linked List

A doubly linked list is a data structure where a set of sequential links of records called nodes exist. Unlike the singly linked list, a node of a doubly linked list consists of three fields: two link fields and one information field. Two link fields provide information about the address of previous nodes and the next nodes in the sequence and one data field.

The link fields are also known as “previous” and “next” pointers and store the addresses of the previous and next nodes in the list. And the previous pointer of the very first node, as well as the next pointer of the last node points to a Null value or called a sentinel node.

Syntax:

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

Where struct keyword refers to a structure, node is the name of the structure, data is the information field which contains only integer values, *next is the pointer of type structure which holds the address of the next node in the sequential list, and *prev is the pointer of the type structure and hold the address of the previous node in the sequential list.

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