1.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Linked List Data Structure Marcus Biel, Software Craftsman http://www.marcus-biel.com

2.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Terminology First of all, let’s have a look at the term “Linked List”. Why is Linked List actually called Linked List?

3.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Terminology Link For the term “Link” - as an analogy - think of a weblink – when you click on it, it brings you from one website to the next.

4.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Terminology List A list is a collection of related objects. Think of a shopping list - it contains every item that you plan to buy.

5.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Terminology Linked List So a Linked List is a collection of related objects, where a link will bring you from one item to the next item.

6.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Terminology Linked List Technically, in our Linked List, an item or entry of the list…

7.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node … is stored in a so called “Node”.

8.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ 23 Node In our simple example, this entry is the number twenty three.

9.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node 23 Each node has a link or pointer to the next element of the list, the next node.

10.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 23 For every item that we want to add to the Linked List, we create a node and store the item in it.

11.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 23 The first item of the list is usually called the head, the last item of the list the tail of the list.

12.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 233 For every new item, we create a new node…

13.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 2393 … and link to it from the last element of the list.

14.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 So a Linked List is a dynamic data structure, that can hold any number of elements, as long as enough memory is available.

15.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List next ( ) 239342 After the list is created, to navigate through the list we call a function like next() which uses the link to go from one item to the next.

16.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 This type of a Linked List is called a Singly Linked List because the links only go in one direction -

17.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 from the head of the list -

18.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 - to the tail.

19.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 So to navigate to the previous element, for example from the element 42 -

20.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 to the element 9

21.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 you have to go back to the head of the list -

22.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List next ( ) 239342 and you have to call the function next() -

23.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 on every single element -

24.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List next ( ) 239342 until you reach the element 9.

25.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 If the list contains a lot of elements, this may take some time.

26.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 Inserting an element after the current element is relatively easy with a Singly Linked List.

27.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 17 239342 You create a new Node containing the new element.

28.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 17 239342 You link to the new element „17“ from the current element „9”.

29.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 17 239342 You add a link pointing from the new “17” element to the existing “42” element.

30.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 23931742 And you’r done.

31.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 Inserting the same element BEFORE the current element is possible in a Singly Linked List, but usually not very efficient.

32.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 It will require to navigate to the previous element, starting from the beginning of the list, as I showed you before.

33.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Singly Linked List 239342 Removing an element from a Singly Linked List has the same issue – it is possible, but generally not very efficient.

34.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node 23 These operations get much easier, when you add a second link to each node, pointing to the previous element.

35.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node 23 This way you can navigate in both directions of the list.

36.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node 23 However the extra link comes at a cost, of extra memory, as well as time to build the more complex structure.

37.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node 23 If this overhead is justified, I cannot generally answer, as it will differ for each use case. If performance is an issue, I advise you to test different options.

38.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 23 so a Linked List that contains nodes that provide a link to the next and the previous node is called a “Doubly Linked List”.

39.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233 For every element that you add to a Doubly Linked List, you need to create two links, so creating a Doubly Linked List is somewhat more complicated.

40.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 2339 Navigation in both directions in a Doubly Linked List is easier, but at the cost of a more complex structure.

41.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 Based on the two way link structure…

42.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 adding or removing an element before or after the current element is relatively easy.

43.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 Here we will add the element “17” before the current element “9”.

44.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 The back link from element “9” to element “3” is removed…

45.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 and replaced by a back link to the new element “17”.

46.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 From the new element “17” we link to the next element “9”.

47.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 …next, we place a back link from 17 to 3…

48.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 233942 17 The old link from element 3 to 9 is replaced by a link from 3 to 17.

49.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Doubly Linked List 23317942 Removing an element from a Doubly Linked List has the same steps as inserting an element to the list, just in reverse order.

50.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ public class Node { private E item; private Node next; public Node(E element, Node next) { this.item = element; this.next = next; } public E item() { return item; } public Node next() { return next; } […] } Node Let‘s look at a code excerpt of a Node in a Singly Linked List. So you have a method to retrieve the item of a Node, and a method to go to the next Node.

51.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Node public class Node { private E item; private Node previous; private Node next; public Node(Node previous, E element, Node next) { this.item = element; this.next = next; this.previous = previous; } public E item() { return item; } public Node previous() { return previous; } public Node next() { return next; } […] } The Node of a Doubly Linked List is very similar, but a bit more complex. Additionally, you have a reference to the previous Node.

52.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ public class LinkedList { private Node currentNode; private Node head; public E get(int index) {…} public boolean add(E e) {…} public E remove(int index) {…} […] } LinkedList A LinkedList usually contains a link to the head of the List. The methods of the Node class will be used to navigate through the list.

53.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ public class LinkedList { private Node currentNode; private Node head; public E get(int index) {…} public boolean add(E e) {…} public E remove(int index) {…} […] } LinkedList The concrete implementation of methods to get, add or remove an element from the list, depends on the type of the node, but such an implementation detail is usually hidden from a user of the list.

54.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ public class LinkedList { private Node currentNode; private Node head; private Node tail; public E get(int index) {…} public boolean add(E e) {…} public E remove(int index) {…} […] } LinkedList Besides the direct link to the current node of the list and the head of the list, a Linked List may also provide a direct link to the tail of the list. This is common for a Doubly Linked List, but may also be beneficial for a Singly Linked List.

55.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Application Scenarios List Queue Stack Double Ended Queue (Deque) Let’s have a look at different application scenarios for a Linked List. A Linked List can be used to implement data structures like List, Queue, Stack or Double Ended Queue.

56.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Application Scenarios List Queue Stack Double Ended Queue (Deque) When comparing implementations based on a Singly- or a Doubly Linked List, there is no overall winner.

57.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Application Scenarios List Queue Stack Double Ended Queue (Deque) Both have advantages and disadvantages that make them useful for different application scenarios.

58.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Application Scenarios List Queue Stack Double Ended Queue (Deque) Using the example of List, Queue, Stack and Double Ended Queue we will in each case decide for a Singly- or a Doubly Linked based implementation.

59.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ Application Scenarios List Queue Stack Double Ended Queue (Deque) An implementation with other data structures, for example an array, is also possible, but that’s a different story.

60.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios A list usually requires random access to any element of the list, so I would recommend an implementation based on a Doubly Linked List.

61.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios In a so called “first in first out” Queue, new elements are inserted at the tail of the queue and removed from the head of the queue.

62.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios Random access is not required, so I would recommend an implementation based on a Singly Linked List.

63.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios A Stack provides a so called “Last in First out” order. Elements are added and removed from the head of the Stack, which makes it even more simple then a Queue.

64.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios Therefore a Singly Linked List is usually sufficient.

65.  2015, Marcus Biel, http://www.marcus-biel.com/http://www.marcus-biel.com/ List Queue Stack Double Ended Queue (Deque) Application Scenarios A Double Ended Queue or Deque is a very dynamic data structure. It allows to access, add or remove elements from both ends. Therefore I would use a Doubly Linked List to implement a Deque.