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A linked list is a sequence of data structures, which are connected together via links.

Linked List is a sequence of links which contains items. Each link contains a connection to another link. Linked list is the second most-used data structure after array. Following are the important terms to understand the concept of Linked List.

Link − Each link of a linked list can store a data called an element.

Next − Each link of a linked list contains a link to the next link called Next.

Linkedlist − A Linked List contains the connection link to the first link called First.

Linked List Representation
Linked list can be visualized as a chain of nodes, where every node points to the next node.

Linked List
As per the above illustration, following are the important points to be considered.

Linked List contains a link element called first.

Each link carries a data field(s) and a link field called next.

Each link is linked with its next link using its next link.

Last link carries a link as null to mark the end of the list.

Types of Linked List
Following are the various types of linked list.

Simple Linked List − Item navigation is forward only.

Doubly Linked List − Items can be navigated forward and backward.

Circular Linked List − Last item contains link of the first element as next and the first element has a link to the last element as previous.

Basic Operations
Following are the basic operations supported by a list.

Insertion − Adds an element at the beginning of the list.

Deletion − Deletes an element at the beginning of the list.

Display − Displays the complete list.

Search − Searches an element using the given key.

Delete − Deletes an element using the given key.

Insertion Operation
Adding a new node in linked list is a more than one step activity. We shall learn this with diagrams here. First, create a node using the same structure and find the location where it has to be inserted.

Linked List Insertion
Imagine that we are inserting a node B (NewNode), between A (LeftNode) and C (RightNode). Then point B.next to C −

NewNode.next −> RightNode;
It should look like this −

Linked List Insertion
Now, the next node at the left should point to the new node.

LeftNode.next −> NewNode;
Linked List Insertion
This will put the new node in the middle of the two. The new list should look like this −

Linked List Insertion
Similar steps should be taken if the node is being inserted at the beginning of the list. While inserting it at the end, the second last node of the list should point to the new node and the new node will point to NULL.

Deletion Operation
Deletion is also a more than one step process. We shall learn with pictorial representation. First, locate the target node to be removed, by using searching algorithms.

Linked List Deletion
The left (previous) node of the target node now should point to the next node of the target node −

LeftNode.next −> TargetNode.next;
Linked List Deletion
This will remove the link that was pointing to the target node. Now, using the following code, we will remove what the target node is pointing at.

TargetNode.next −> NULL;
Linked List Deletion
We need to use the deleted node. We can keep that in memory otherwise we can simply deallocate memory and wipe off the target node completely.

Linked List Deletion
Reverse Operation
This operation is a thorough one. We need to make the last node to be pointed by the head node and reverse the whole linked list.

Linked List Reverse Operation
First, we traverse to the end of the list. It should be pointing to NULL. Now, we shall make it point to its previous node −

Linked List Reverse Operation
We have to make sure that the last node is not the last node. So we'll have some temp node, which looks like the head node pointing to the last node. Now, we shall make all left side nodes point to their previous nodes one by one.

Linked List Reverse Operation
Except the node (first node) pointed by the head node, all nodes should point to their predecessor, making them their new successor. The first node will point to NULL.

Linked List Reverse Operation
We'll make the head node point to the new first node by using the temp node.

Doubly Linked List is a variation of Linked list in which navigation is possible in both ways, either forward and backward easily as compared to Single Linked List. Following are the important terms to understand the concept of doubly linked list.

Link − Each link of a linked list can store a data called an element.

Next − Each link of a linked list contains a link to the next link called Next.

Prev − Each link of a linked list contains a link to the previous link called Prev.

LinkedList − A Linked List contains the connection link to the first link called First and to the last link called Last.

Doubly Linked List Representation
Doubly Linked List
As per the above illustration, following are the important points to be considered.

Doubly Linked List contains a link element called first and last.

Each link carries a data field(s) and two link fields called next and prev.

Each link is linked with its next link using its next link.

Each link is linked with its previous link using its previous link.

The last link carries a link as null to mark the end of the list.

Basic Operations
Following are the basic operations supported by a list.

Insertion − Adds an element at the beginning of the list.

Deletion − Deletes an element at the beginning of the list.

Insert Last − Adds an element at the end of the list.

Delete Last − Deletes an element from the end of the list.

Insert After − Adds an element after an item of the list.

Delete − Deletes an element from the list using the key.

Display forward − Displays the complete list in a forward manner.

Display backward − Displays the complete list in a backward manner.

Insertion Operation
Following code demonstrates the insertion operation at the beginning of a doubly linked list.

Example
//insert link at the first location
void insertFirst(int key, int data) {

//create a link
struct node link = (struct node) malloc(sizeof(struct node));
link->key = key;
link->data = data;

if(isEmpty()) {
//make it the last link
last = link;
} else {
//update first prev link
head->prev = link;
}

//point it to old first link
link->next = head;

//point first to new first link
head = link;
}
Deletion Operation
Following code demonstrates the deletion operation at the beginning of a doubly linked list.

Example
//delete first item
struct node* deleteFirst() {

//save reference to first link
struct node *tempLink = head;

//if only one link
if(head->next == NULL) {
last = NULL;
} else {
head->next->prev = NULL;
}

head = head->next;

//return the deleted link
return tempLink;
}
Insertion at the End of an Operation
Following code demonstrates the insertion operation at the last position of a doubly linked list.

Example
//insert link at the last location
void insertLast(int key, int data) {

//create a link
struct node link = (struct node) malloc(sizeof(struct node));
link->key = key;
link->data = data;

if(isEmpty()) {
//make it the last link
last = link;
} else {
//make link a new last link
last->next = link;

  //mark old last node as prev of new link
  link->prev = last;

}

//point last to new last node
last = link;
}