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• A Linked List is a linear data structure that is used to store a collection of data with the help of nodes. A linked list is made up of two items that are data and a reference to the next node. A reference to the next node is given with the help of pointers and data is the value of a node. Each node contains data and links to the other nodes. It is an ordered collection of data elements called a node and the linear order is maintained by pointers. It has an upper hand over the array as the number of nodes i.e. the size of the linked list is not fixed and can grow and shrink as and when required, unlike arrays. Some of the features of the linked list are as follows:
• The consecutive elements are connected by pointers.
• The size of a linked list is not fixed.
• The last node of the linked list points to null.
• Memory is not wasted but extra memory is consumed as it also uses pointers to keep track of the next successive node.
• The entry point of a linked list is known as the head.

The various types of linked lists are as follows:

• Singly Linked List: It is the most basic linked list in which traversal is unidirectional i.e. from the head node to the last node.

Example:

C++

 `class` `Node {``public``:``    ``int` `data;``    ``Node* next;` `    ``Node(``int` `data) {``        ``this``->data = data;``        ``this``->next = nullptr;``    ``}``};` `class` `SinglyLinkedList {``public``:``    ``Node* head;` `    ``SinglyLinkedList() {``        ``this``->head = nullptr;``    ``}` `    ``void` `add_node(``int` `data) {``        ``Node* new_node = ``new` `Node(data);``        ``new_node->next = ``this``->head;``        ``this``->head = new_node;``    ``}``};`

Python

 `class` `Node:``  ``def` `__init__(``self``, data):``    ``self``.data ``=` `data``    ``self``.``next` `=` `None` `class` `SinglyLinkedList:``  ``def` `__init__(``self``):``    ``self``.head ``=` `None``    ` `  ``def` `add_node(``self``, data):``    ``new_node ``=` `Node(data)``    ``new_node.``next` `=` `self``.head``    ``self``.head ``=` `new_node`

Java

 `class` `Node {``    ``int` `data;``    ``Node next;` `    ``Node(``int` `d) {``        ``data = d;``        ``next = ``null``;``    ``}``}` `class` `SinglyLinkedList {``    ``Node head;` `    ``SinglyLinkedList() {``        ``head = ``null``;``    ``}` `    ``void` `add_node(``int` `data) {``        ``Node new_node = ``new` `Node(data);``        ``new_node.next = head;``        ``head = new_node;``    ``}``}`

C#

 `using` `System;` `public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node next;` `    ``public` `Node(``int` `d) {``        ``data = d;``        ``next = ``null``;``    ``}``}` `public` `class` `SinglyLinkedList {``    ``public` `Node head;` `    ``public` `SinglyLinkedList() {``        ``head = ``null``;``    ``}` `    ``public` `void` `add_node(``int` `data) {``        ``Node new_node = ``new` `Node(data);``        ``new_node.next = head;``        ``head = new_node;``    ``}``}`

Javascript

 `class Node {``  ``constructor(data) {``    ``this``.data = data;``    ``this``.next = ``null``;``  ``}``}` `class SinglyLinkedList {``  ``constructor() {``    ``this``.head = ``null``;``  ``}` `  ``add_node(data) {``    ``let new_node = ``new` `Node(data);``    ``new_node.next = ``this``.head;``    ``this``.head = new_node;``  ``}``}`

• Doubly Linked List: In this linked list, traversal can be done in both ways, and hence it requires an extra pointer.

Example:

C

 `#include ``#include ` `struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``    ``struct` `Node* prev;``};` `struct` `DoublyLinkedList {``    ``struct` `Node* head;``};` `void` `addNode(``struct` `DoublyLinkedList* list, ``int` `data) {``    ``struct` `Node* newNode = (``struct` `Node*) ``malloc``(``sizeof``(``struct` `Node));``    ``newNode->data = data;``    ``newNode->next = list->head;``    ``newNode->prev = NULL;``    ``if` `(list->head != NULL) {``        ``list->head->prev = newNode;``    ``}``    ``list->head = newNode;``}`

C++

 `class` `Node {``public``:``int` `data;``Node* next;``Node* prev;``Node(``int` `d) : data(d), next(nullptr), prev(nullptr) {}``};` `class` `DoublyLinkedList {``public``:``Node* head;``DoublyLinkedList() : head(nullptr) {}` `void` `add_node(``int` `data) {``    ``Node* new_node = ``new` `Node(data);``    ``new_node->next = head;``    ``if` `(head != nullptr) {``        ``head->prev = new_node;``    ``}``    ``head = new_node;``}``};`

Python

 `class` `Node:``  ``def` `__init__(``self``, data):``    ``self``.data ``=` `data``    ``self``.``next` `=` `None``    ``self``.prev ``=` `None` `class` `DoublyLinkedList:``  ``def` `__init__(``self``):``    ``self``.head ``=` `None``    ` `  ``def` `add_node(``self``, data):``    ``new_node ``=` `Node(data)``    ``new_node.``next` `=` `self``.head``    ``if` `self``.head ``is` `not` `None``:``      ``self``.head.prev ``=` `new_node``    ``self``.head ``=` `new_node`

Java

 `public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node next;``    ``public` `Node prev;``    ` `    ``public` `Node(``int` `d) {``        ``data = d;``        ``next = ``null``;``        ``prev = ``null``;``    ``}``}` `public` `class` `DoublyLinkedList {``    ``public` `Node head;``    ` `    ``public` `DoublyLinkedList() {``        ``head = ``null``;``    ``}``    ` `    ``public` `void` `addNode(``int` `data) {``        ``Node newNode = ``new` `Node(data);``        ``newNode.next = head;``        ``if` `(head != ``null``) {``            ``head.prev = newNode;``        ``}``        ``head = newNode;``    ``}``}`

C#

 `public` `class` `Node {``    ``public` `int` `data;``    ``public` `Node next;``    ``public` `Node prev;``    ` `    ``public` `Node(``int` `d) {``        ``data = d;``        ``next = ``null``;``        ``prev = ``null``;``    ``}``}` `public` `class` `DoublyLinkedList {``    ``public` `Node head;``    ` `    ``public` `DoublyLinkedList() {``        ``head = ``null``;``    ``}``    ` `    ``public` `void` `AddNode(``int` `data) {``        ``Node newNode = ``new` `Node(data);``        ``newNode.next = head;``        ``if` `(head != ``null``) {``            ``head.prev = newNode;``        ``}``        ``head = newNode;``    ``}``}`

Javascript

 `class Node {``  ``constructor(d) {``    ``this``.data = d;``    ``this``.next = ``null``;``    ``this``.prev = ``null``;``  ``}``}` `class DoublyLinkedList {``  ``constructor() {``    ``this``.head = ``null``;``  ``}``  ` `  ``addNode(data) {``    ``const newNode = ``new` `Node(data);``    ``newNode.next = ``this``.head;``    ``if` `(``this``.head !== ``null``) {``      ``this``.head.prev = newNode;``    ``}``    ``this``.head = newNode;``  ``}``}`

• Circular Linked List: This linked list is unidirectional but in this list, the last node points to the first i.e. the head node and hence it becomes circular in nature.

Example:

C

 `struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `struct` `CircularLinkedList {``    ``struct` `Node* head;``};` `void` `add_node(``struct` `CircularLinkedList* list, ``int` `data) {``    ``struct` `Node* new_node = (``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));``    ``new_node->data = data;``    ``new_node->next = NULL;` `    ``if` `(list->head == NULL) {``        ``list->head = new_node;``        ``new_node->next = new_node;``        ``return``;``    ``}` `    ``struct` `Node* current = list->head;` `    ``while` `(current->next != list->head) {``        ``current = current->next;``    ``}` `    ``current->next = new_node;``    ``new_node->next = list->head;``}`

C++

 `class` `Node {``public``:``int` `data;``Node* next;``Node(``int` `d) : data(d), next(nullptr) {}``};` `class` `CircularLinkedList {``public``:``Node* head;``CircularLinkedList() : head(nullptr) {}` `void` `add_node(``int` `data) {``    ``Node* new_node = ``new` `Node(data);``    ``if` `(head == nullptr) {``        ``head = new_node;``        ``new_node->next = head;``        ``return``;``    ``}``    ``Node* current = head;``    ``while` `(current->next != head) {``        ``current = current->next;``    ``}``    ``current->next = new_node;``    ``new_node->next = head;``}``};`

Java

 `class` `Node {``    ``public` `int` `data;``    ``public` `Node next;` `    ``public` `Node(``int` `d) {``        ``data = d;``        ``next = ``null``;``    ``}``}` `class` `CircularLinkedList {``    ``public` `Node head;` `    ``public` `CircularLinkedList() {``        ``head = ``null``;``    ``}` `    ``public` `void` `add_node(``int` `data) {``        ``Node new_node = ``new` `Node(data);``        ``if` `(head == ``null``) {``            ``head = new_node;``            ``new_node.next = head;``            ``return``;``        ``}``        ``Node current = head;``        ``while` `(current.next != head) {``            ``current = current.next;``        ``}``        ``current.next = new_node;``        ``new_node.next = head;``    ``}``}`

Python

 `class` `Node:``  ``def` `__init__(``self``, data):``    ``self``.data ``=` `data``    ``self``.``next` `=` `None` `class` `CircularLinkedList:``  ``def` `__init__(``self``):``    ``self``.head ``=` `None``    ` `  ``def` `add_node(``self``, data):``    ``new_node ``=` `Node(data)``    ``if` `self``.head ``is` `None``:``      ``self``.head ``=` `new_node``      ``new_node.``next` `=` `self``.head``      ``return``    ``current ``=` `self``.head``    ``while` `current.``next` `!``=` `self``.head:``      ``current ``=` `current.``next``    ``current.``next` `=` `new_node``    ``new_node.``next` `=` `self``.head`

Javascript

 `class Node {``  ``constructor(data) {``    ``this``.data = data;``    ``this``.next = ``null``;``  ``}``}` `class CircularLinkedList {``  ``constructor() {``    ``this``.head = ``null``;``  ``}` `  ``add_node(data) {``    ``const new_node = ``new` `Node(data);``    ``if` `(``this``.head === ``null``) {``      ``this``.head = new_node;``      ``new_node.next = ``this``.head;``      ``return``;``    ``}``    ``let current = ``this``.head;``    ``while` `(current.next !== ``this``.head) {``      ``current = current.next;``    ``}``    ``current.next = new_node;``    ``new_node.next = ``this``.head;``  ``}``}`

C#

 `public` `class` `Node``{``    ``public` `int` `data;``    ``public` `Node next;` `    ``public` `Node(``int` `d)``    ``{``        ``data = d;``        ``next = ``null``;``    ``}``}` `public` `class` `CircularLinkedList``{``    ``public` `Node head;` `    ``public` `CircularLinkedList()``    ``{``        ``head = ``null``;``    ``}` `    ``public` `void` `add_node(``int` `data)``    ``{``        ``Node new_node = ``new` `Node(data);` `        ``if` `(head == ``null``)``        ``{``            ``head = new_node;``            ``new_node.next = head;``            ``return``;``        ``}` `        ``Node current = head;` `        ``while` `(current.next != head)``        ``{``            ``current = current.next;``        ``}` `        ``current.next = new_node;``        ``new_node.next = head;``    ``}``}`

• Circular Doubly Linked List: The circular doubly linked list is a combination of the doubly linked list and the circular linked list. It means that this linked list is bidirectional and contains two pointers and the last pointer points to the first pointer.

Linked Lists are most commonly used for:

• Linked Lists are used to implement stacks and queues.
• It is used for the various representations of trees and graphs.
• It is used in dynamic memory allocation( linked list of free blocks).
• It is used for representing sparse matrices.
• It is used for the manipulation of polynomials.
• It is also used for performing arithmetic operations on long integers.
• It is used for finding paths in networks.
• In operating systems, they can be used in Memory management, process scheduling and file system.
• Linked lists can be used to improve the performance of algorithms that need to frequently insert or delete items from large collections of data.
• Implementing algorithms such as the LRU cache, which uses a linked list to keep track of the most recently used items in a cache.

Applications of Linked Lists in real world:

• The list of songs in the music player are linked to the previous and next songs.
• In a web browser, previous and next web page URLs are linked through the previous and next buttons.
• In image viewer, the previous and next images are linked with the help of the previous and next buttons.
• Switching between two applications is carried out by using “alt+tab” in windows and “cmd+tab” in mac book. It requires the functionality of circular linked list.
• In mobile phones, we save the contacts of the people. The newly entered contact details will be placed at the correct alphabetical order. This can be achieved by linked list to set contact at correct alphabetical position.
• The modifications that we make in documents are actually created as nodes in doubly linked list. We can simply use the undo option by pressing Ctrl+Z to modify the contents. It is done by the functionality of linked list.

Linked lists are a popular data structure in computer science, and have a number of advantages over other data structures, such as arrays. Some of the key advantages of linked lists are:

• Dynamic size: Linked lists do not have a fixed size, so you can add or remove elements as needed, without having to worry about the size of the list. This makes linked lists a great choice when you need to work with a collection of items whose size can change dynamically.
• Efficient Insertion and Deletion: Inserting or deleting elements in a linked list is fast and efficient, as you only need to modify the reference of the next node, which is an O(1) operation.
• Memory Efficiency: Linked lists use only as much memory as they need, so they are more efficient with memory compared to arrays, which have a fixed size and can waste memory if not all elements are used.
• Easy to Implement: Linked lists are relatively simple to implement and understand compared to other data structures like trees and graphs.
• Flexibility: Linked lists can be used to implement various abstract data types, such as stacks, queues, and associative arrays.
• Easy to navigate: Linked lists can be easily traversed, making it easier to find specific elements or perform operations on the list.

In conclusion, linked lists are a powerful and flexible data structure that have a number of advantages over other data structures, making them a great choice for solving many data structure and algorithm problems.