Doubly Linked List | Set 1 [Introduction and Insertion]
We strongly recommend to refer following post as a prerequisite of this post.
Linked List Introduction
Inserting a node in Singly Linked List
A Doubly Linked List [DLL] contains an extra pointer, typically called previous pointer, together with next pointer and data which are there in singly linked list.
Following is representation of a DLL node in C language.
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/* 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
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/* 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
};
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// 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; }
}
}
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# 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
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// 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
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// Class for Doubly Linked List
var head; // head of list
/* Doubly Linked list Node */
class Node {
// Constructor to create a new node
// next and prev is by default initialized as null
constructor[val] {
this.data = val;
this.prev = null;
this.next = null;
}
}
// This code contributed by gauravrajput1
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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.
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]
Following are the 5 steps to add node at the front.
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/* 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
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/* 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;
}
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// 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;
}
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# 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
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// 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
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// 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.
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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.
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/* 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]
{
coutnext = 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.
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/* 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;
}
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/* 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;
}
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# 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
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/* 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
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function InsertAfter[prev_Node,new_data]
{
/*1. check if the given prev_node is NULL */
if [prev_Node == null] {
document.write["The given previous node cannot be NULL
"];
return;
}
/* 2. allocate node
* 3. put in the data */
let 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 unknown2108
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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.
Following are the 7 steps to add node at the end.
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/* 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
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/* 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;
}
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// 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;
}
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# 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
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// 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
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// Add a node at the end of the list
function append[new_data]
{
/* 1. allocate node
* 2. put in the data */
var new_node = new Node[new_data];
var 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 Rajput-Ji
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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.
- 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
- Allocate memory for the new node, let it be called new_node
- Set new_node->data = new_data
- Set the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prev
- Set the previous pointer of the next_node as the new_node, next_node->prev = new_node
- Set the next pointer of this new_node as the next_node, new_node->next = next_node;
- 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
- Else, if the prev of new_node is NULL, it will be the new head node. So, make [*head_ref] = new_node.
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.
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// 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]
{
coutnext = 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;
cout8->6->4->NULL
insertAfter[head->next, 8];
cout 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;
}
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// 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
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// A complete working javascript program to demonstrate all
// Class for Doubly Linked List
var head; // head of list
/* Doubly Linked list Node */
class Node {
// Constructor to create a new node
// next and prev is by default initialized as null
constructor[d] {
this.data = d;
this.next = null;
this.prev = null;
}
}
// Adding a node at the front of the list
function push[new_data] {
/*
* 1. allocate node 2. put in the data
*/
var 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
function InsertBefore[next_node , new_data] {
/* Check if the given nx_node is NULL */
if [next_node == null] {
document.write["The given next node can not be NULL"];
return;
}
// Allocate node, put in the data
var 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
*/
function InsertAfter[prev_Node , new_data] {
/* 1. check if the given prev_node is NULL */
if [prev_Node == null] {
document.write["The given previous node cannot be NULL "];
return;
}
/*
* 2. allocate node 3. put in the data
*/
var 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
function append[new_data] {
/*
* 1. allocate node 2. put in the data
*/
var new_node = new Node[new_data];
var 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
function printlist[node] {
var last = null;
document.write["
Traversal in forward Direction "];
while [node != null] {
document.write[node.data + " "];
last = node;
node = node.next;
}
document.write[];
document.write["
Traversal in reverse direction "];
while [last != null] {
document.write[last.data + " "];
last = last.prev;
}
}
/* Driver program to test above functions */
/* Start with the empty list */
// Insert 6. So linked list becomes 6->NULL
append[6];
// Insert 7 at the beginning. So
// linked list becomes 7->6->NULL
push[7];
// Insert 1 at the beginning. So
// linked list becomes 1->7->6->NULL
push[1];
// Insert 4 at the end. So linked
// list becomes 1->7->6->4->NULL
append[4];
// Insert 8, after 7. So linked
// list becomes 1->7->8->6->4->NULL
InsertAfter[head.next, 8];
// Insert 5, before 8.So linked
// list becomes 1->7->5->8->6->4
InsertBefore[head.next.next, 5];
document.write["Created DLL is:
"];
printlist[head];
// This code is contributed by Rajput-Ji
|
Output:
Created DLL is: Traversal in forward Direction 1 7 5 8 6 4 Traversal in reverse direction 4 6 8 5 7 1Also 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.
Insert value in sorted way in a sorted doubly linked list
Given a sorted doubly linked list and a value to insert, write a function to insert the value in sorted way.
Initial doubly linked list
Doubly Linked List after insertion of 9
Introduction to Singly Linked List
Singly Linked List is a variant of Linked List which allows only forward traversal of linked lists. This is a simple form, yet it is effective for several problems such as Big Integer calculations.
A singly linked list is made up of nodes where each node has two parts:
- The first part contains the actual data of the node
- The second part contains a link that points to the next node of the list that is the address of the next node.
The beginning of the node marked by a special pointer named START. The pointer points to the fist node of the list but the link part of the last node has no next node to point to.
The main difference from an array is:
- Elements are not stored in contiguous memory locations.
- Size of Linked List need not be known in advance. It can increase at runtime depending on number of elements dynamically without any overhead.
In Singly Linked List, only the pointer to the first node is stored. The other nodes are accessed one by one.
To get the address of ith node, we need to traverse all nodes before it because the address of ith node is stored with i-1th node and so on.
Singly Linked List vs Doubly Linked List
Before looking at the differences between the singly linked list and doubly linked list, we first understand what is singly linked list and doubly linked list separately.
Circular Doubly Linked List
Created: March-09, 2021 | Updated: June-17, 2021
A Circular Doubly Linked List is a combination of both the circular linked list and doubly linked list. Its two nodes are connected by both the previous and next pointer. The last node’s next pointer points to the first node and the first node’s previous pointer points to the last node. It can be traversed from both directions and jumping from tail to head or from head to tail. It is also used for the implementation of advanced data structures like Fibonacci Heap.
Basic concepts and nomenclatureEdit
Each record of a linked list is often called an 'element' or 'node'.
The field of each node that contains the address of the next node is usually called the 'next link' or 'next pointer'. The remaining fields are known as the 'data', 'information', 'value', 'cargo', or 'payload' fields.
The 'head' of a list is its first node. The 'tail' of a list may refer either to the rest of the list after the head, or to the last node in the list. In Lisp and some derived languages, the next node may be called the 'cdr' [pronounced could-er] of the list, while the payload of the head node may be called the 'car'.
Singly linked listEdit
Singly linked lists contain nodes which have a data field as well as 'next' field, which points to the next node in line of nodes. Operations that can be performed on singly linked lists include insertion, deletion and traversal.
A singly linked list whose nodes contain two fields: an integer value and a link to the next node
The following code demonstrates how to add a new node with data "value" to the end of a singly linked list:
Doubly linked listEdit
In a 'doubly linked list', each node contains, besides the next-node link, a second link field pointing to the 'previous' node in the sequence. The two links may be called 'forward['s'] and 'backwards', or 'next' and 'prev'['previous'].
A doubly linked list whose nodes contain three fields: an integer value, the link forward to the next node, and the link backward to the previous node
A technique known as XOR-linking allows a doubly linked list to be implemented using a single link field in each node. However, this technique requires the ability to do bit operations on addresses, and therefore may not be available in some high-level languages.
Many modern operating systems use doubly linked lists to maintain references to active processes, threads, and other dynamic objects.[2] A common strategy for rootkits to evade detection is to unlink themselves from these lists.[3]
Multiply linked listEdit
In a 'multiply linked list', each node contains two or more link fields, each field being used to connect the same set of data records in a different order of same set [e.g., by name, by department, by date of birth, etc.]. While doubly linked lists can be seen as special cases of multiply linked list, the fact that the two and more orders are opposite to each other leads to simpler and more efficient algorithms, so they are usually treated as a separate case.
Circular linked listEdit
In the last node of a list, the link field often contains a null reference, a special value is used to indicate the lack of further nodes. A less common convention is to make it point to the first node of the list; in that case, the list is said to be 'circular' or 'circularly linked'; otherwise, it is said to be 'open' or 'linear'. It is a list where the last pointer points to the first node.
In the case of a circular doubly linked list, the first node also points to the last node of the list.
Sentinel nodesEdit
In some implementations an extra 'sentinel' or 'dummy' node may be added before the first data record or after the last one. This convention simplifies and accelerates some list-handling algorithms, by ensuring that all links can be safely dereferenced and that every list [even one that contains no data elements] always has a "first" and "last" node.
Empty listsEdit
An empty list is a list that contains no data records. This is usually the same as saying that it has zero nodes. If sentinel nodes are being used, the list is usually said to be empty when it has only sentinel nodes.
Hash linkingEdit
The link fields need not be physically part of the nodes. If the data records are stored in an array and referenced by their indices, the link field may be stored in a separate array with the same indices as the data records.
List handlesEdit
Since a reference to the first node gives access to the whole list, that reference is often called the 'address', 'pointer', or 'handle' of the list. Algorithms that manipulate linked lists usually get such handles to the input lists and return the handles to the resulting lists. In fact, in the context of such algorithms, the word "list" often means "list handle". In some situations, however, it may be convenient to refer to a list by a handle that consists of two links, pointing to its first and last nodes.
Combining alternativesEdit
The alternatives listed above may be arbitrarily combined in almost every way, so one may have circular doubly linked lists without sentinels, circular singly linked lists with sentinels, etc.
TradeoffsEdit
As with most choices in computer programming and design, no method is well suited to all circumstances. A linked list data structure might work well in one case, but cause problems in another. This is a list of some of the common tradeoffs involving linked list structures.
Linked lists vs. dynamic arraysEdit
A dynamic array is a data structure that allocates all elements contiguously in memory, and keeps a count of the current number of elements. If the space reserved for the dynamic array is exceeded, it is reallocated and [possibly] copied, which is an expensive operation.
Linked lists have several advantages over dynamic arrays. Insertion or deletion of an element at a specific point of a list, assuming that we have indexed a pointer to the node [before the one to be removed, or before the insertion point] already, is a constant-time operation [otherwise without this reference it is O[n]], whereas insertion in a dynamic array at random locations will require moving half of the elements on average, and all the elements in the worst case. While one can "delete" an element from an array in constant time by somehow marking its slot as "vacant", this causes fragmentation that impedes the performance of iteration.
Moreover, arbitrarily many elements may be inserted into a linked list, limited only by the total memory available; while a dynamic array will eventually fill up its underlying array data structure and will have to reallocate—an expensive operation, one that may not even be possible if memory is fragmented, although the cost of reallocation can be averaged over insertions, and the cost of an insertion due to reallocation would still be amortized O[1]. This helps with appending elements at the array's end, but inserting into [or removing from] middle positions still carries prohibitive costs due to data moving to maintain contiguity. An array from which many elements are removed may also have to be resized in order to avoid wasting too much space.
On the other hand, dynamic arrays [as well as fixed-size array data structures] allow constant-time random access, while linked lists allow only sequential access to elements. Singly linked lists, in fact, can be easily traversed in only one direction. This makes linked lists unsuitable for applications where it's useful to look up an element by its index quickly, such as heapsort. Sequential access on arrays and dynamic arrays is also faster than on linked lists on many machines, because they have optimal locality of reference and thus make good use of data caching.
Another disadvantage of linked lists is the extra storage needed for references, which often makes them impractical for lists of small data items such as characters or boolean values, because the storage overhead for the links may exceed by a factor of two or more the size of the data. In contrast, a dynamic array requires only the space for the data itself [and a very small amount of control data].[note 1] It can also be slow, and with a naïve allocator, wasteful, to allocate memory separately for each new element, a problem generally solved using memory pools.
Some hybrid solutions try to combine the advantages of the two representations. Unrolled linked lists store several elements in each list node, increasing cache performance while decreasing memory overhead for references. CDR coding does both these as well, by replacing references with the actual data referenced, which extends off the end of the referencing record.
A good example that highlights the pros and cons of using dynamic arrays vs. linked lists is by implementing a program that resolves the Josephus problem. The Josephus problem is an election method that works by having a group of people stand in a circle. Starting at a predetermined person, one may count around the circle n times. Once the nth person is reached, one should remove them from the circle and have the members close the circle. The process is repeated until only one person is left. That person wins the election. This shows the strengths and weaknesses of a linked list vs. a dynamic array, because if the people are viewed as connected nodes in a circular linked list, then it shows how easily the linked list is able to delete nodes [as it only has to rearrange the links to the different nodes]. However, the linked list will be poor at finding the next person to remove and will need to search through the list until it finds that person. A dynamic array, on the other hand, will be poor at deleting nodes [or elements] as it cannot remove one node without individually shifting all the elements up the list by one. However, it is exceptionally easy to find the nth person in the circle by directly referencing them by their position in the array.
The list ranking problem concerns the efficient conversion of a linked list representation into an array. Although trivial for a conventional computer, solving this problem by a parallel algorithm is complicated and has been the subject of much research.
A balanced tree has similar memory access patterns and space overhead to a linked list while permitting much more efficient indexing, taking O[log n] time instead of O[n] for a random access. However, insertion and deletion operations are more expensive due to the overhead of tree manipulations to maintain balance. Schemes exist for trees to automatically maintain themselves in a balanced state: AVL trees or red–black trees.
Singly linked linear lists vs. other listsEdit
While doubly linked and circular lists have advantages over singly linked linear lists, linear lists offer some advantages that make them preferable in some situations.
A singly linked linear list is a recursive data structure, because it contains a pointer to a smaller object of the same type. For that reason, many operations on singly linked linear lists [such as merging two lists, or enumerating the elements in reverse order] often have very simple recursive algorithms, much simpler than any solution using iterative commands. While those recursive solutions can be adapted for doubly linked and circularly linked lists, the procedures generally need extra arguments and more complicated base cases.
Linear singly linked lists also allow tail-sharing, the use of a common final portion of sub-list as the terminal portion of two different lists. In particular, if a new node is added at the beginning of a list, the former list remains available as the tail of the new one—a simple example of a persistent data structure. Again, this is not true with the other variants: a node may never belong to two different circular or doubly linked lists.
In particular, end-sentinel nodes can be shared among singly linked non-circular lists. The same end-sentinel node may be used for every such list. In Lisp, for example, every proper list ends with a link to a special node, denoted by nil or [], whose CAR and CDR links point to itself. Thus a Lisp procedure can safely take the CAR or CDR of any list.
The advantages of the fancy variants are often limited to the complexity of the algorithms, not in their efficiency. A circular list, in particular, can usually be emulated by a linear list together with two variables that point to the first and last nodes, at no extra cost.
Doubly linked vs. singly linkedEdit
Double-linked lists require more space per node [unless one uses XOR-linking], and their elementary operations are more expensive; but they are often easier to manipulate because they allow fast and easy sequential access to the list in both directions. In a doubly linked list, one can insert or delete a node in a constant number of operations given only that node's address. To do the same in a singly linked list, one must have the address of the pointer to that node, which is either the handle for the whole list [in case of the first node] or the link field in the previous node. Some algorithms require access in both directions. On the other hand, doubly linked lists do not allow tail-sharing and cannot be used as persistent data structures.
Circularly linked vs. linearly linkedEdit
A circularly linked list may be a natural option to represent arrays that are naturally circular, e.g. the corners of a polygon, a pool of buffers that are used and released in FIFO ["first in, first out"] order, or a set of processes that should be time-shared in round-robin order. In these applications, a pointer to any node serves as a handle to the whole list.
With a circular list, a pointer to the last node gives easy access also to the first node, by following one link. Thus, in applications that require access to both ends of the list [e.g., in the implementation of a queue], a circular structure allows one to handle the structure by a single pointer, instead of two.
A circular list can be split into two circular lists, in constant time, by giving the addresses of the last node of each piece. The operation consists in swapping the contents of the link fields of those two nodes. Applying the same operation to any two nodes in two distinct lists joins the two list into one. This property greatly simplifies some algorithms and data structures, such as the quad-edge and face-edge.
The simplest representation for an empty circular list [when such a thing makes sense] is a null pointer, indicating that the list has no nodes. Without this choice, many algorithms have to test for this special case, and handle it separately. By contrast, the use of null to denote an empty linear list is more natural and often creates fewer special cases.
For some applications, it can be useful to use singly linked lists that can vary between being circular and being linear, or even circular with a linear initial segment. Algorithms for searching or otherwise operating on these have to take precautions to avoid accidentally entering an endless loop. One usual method is to have a second pointer walking the list at half or double the speed, and if both pointers meet at the same node, you know you found a cycle.
Using sentinel nodesEdit
Sentinel node may simplify certain list operations, by ensuring that the next or previous nodes exist for every element, and that even empty lists have at least one node. One may also use a sentinel node at the end of the list, with an appropriate data field, to eliminate some end-of-list tests. For example, when scanning the list looking for a node with a given value x, setting the sentinel's data field to x makes it unnecessary to test for end-of-list inside the loop. Another example is the merging two sorted lists: if their sentinels have data fields set to +∞, the choice of the next output node does not need special handling for empty lists.
However, sentinel nodes use up extra space [especially in applications that use many short lists], and they may complicate other operations [such as the creation of a new empty list].
However, if the circular list is used merely to simulate a linear list, one may avoid some of this complexity by adding a single sentinel node to every list, between the last and the first data nodes. With this convention, an empty list consists of the sentinel node alone, pointing to itself via the next-node link. The list handle should then be a pointer to the last data node, before the sentinel, if the list is not empty; or to the sentinel itself, if the list is empty.
The same trick can be used to simplify the handling of a doubly linked linear list, by turning it into a circular doubly linked list with a single sentinel node. However, in this case, the handle should be a single pointer to the dummy node itself.[8]