Linked List data structure
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Transcript of Linked List data structure
2015, Marcus Biel, http://www.marcus-biel.com/
Linked ListData Structure
Marcus Biel, Software Craftsmanhttp://www.marcus-biel.com
2015, Marcus Biel, 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?
2015, Marcus Biel, http://www.marcus-biel.com/
TerminologyLink
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.
2015, Marcus Biel, http://www.marcus-biel.com/
TerminologyList
A list is a collection of related objects. Think of a shopping list - it contains every item that you plan to buy.
2015, Marcus Biel, http://www.marcus-biel.com/
TerminologyLinked List
So a Linked List is a collection of related objects, where a link will bring you from one item to the next item.
2015, Marcus Biel, http://www.marcus-biel.com/
TerminologyLinked List
Technically, in our Linked List, an item or entry of the list…
2015, Marcus Biel, http://www.marcus-biel.com/
Node
… is stored in a so called “Node”.
2015, Marcus Biel, http://www.marcus-biel.com/
23
Node
In our simple example, this entry is the number twenty three.
2015, Marcus Biel, http://www.marcus-biel.com/
Node23
Each node has a link or pointer to the next element of the list, the next node.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23
For every item that we want to add to the Linked List, we create a node and store the item in it.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23
The first item of the list is usually called the head, the last item of the list the tail of the list.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 3
For every new item, we create a new node…
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93
… and link to it from the last element of the list.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
So a Linked List is a dynamic data structure, that can hold any number of elements, as long as enough memory is available.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
next ( )
23 93 42
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.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
This type of a Linked List is called a Singly Linked List because the links only go in one direction -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
from the head of the list -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
- to the tail.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
So to navigate to the previous element, for example from the element 42 -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
to the element 9
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Singly Linked List23 93 42
you have to go back to the head of the list -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
next ( )
23 93 42
and you have to call the function next() -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
on every single element -
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
next ( )
23 93 42
until you reach the element 9.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
If the list contains a lot of elements, this may take some time.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
Inserting an element after the current element is relatively easy with a Singly Linked List.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
17
23 93 42
You create a new Node containing the new element.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
17
23 93 42
You link to the new element „17“ from the current element „9”.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List
17
23 93 42
You add a link pointing from the new “17” element to the existing “42” element.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 17 42
And you’r done.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
Inserting the same element BEFORE the current element is possible in a Singly Linked List, but usually not very efficient.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
It will require to navigate to the previous element, starting from the beginning of the list, as I showed you before.
2015, Marcus Biel, http://www.marcus-biel.com/
Singly Linked List23 93 42
Removing an element from a Singly Linked List has the same issue –
it is possible, but generally not very efficient.
2015, Marcus Biel, http://www.marcus-biel.com/
Node23
These operations get much easier, when you add a second link to each node,
pointing to the previous element.
2015, Marcus Biel, http://www.marcus-biel.com/
Node23
This way you can navigate in both directions of the list.
2015, Marcus Biel, http://www.marcus-biel.com/
Node23
However the extra link comes at a cost, of extra memory, as well as time to build the more complex structure.
2015, Marcus Biel, http://www.marcus-biel.com/
Node23
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.
2015, Marcus Biel, 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”.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3
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.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9
Navigation in both directions in a Doubly Linked List is easier, but at the cost of a more complex structure.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
Based on the two way link structure…
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
adding or removing an element before or
after the current element is relatively easy.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
Here we will add the element “17” before the current element “9”.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
The back link from element “9” to element “3” is removed…
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
and replaced by a back link to the new element “17”.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
From the new element “17” we link to the next element “9”.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
…next, we place a back link from 17 to 3…
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 9 42
17
The old link from element 3 to 9 is replaced by a link from 3 to 17.
2015, Marcus Biel, http://www.marcus-biel.com/
Doubly Linked List
23 3 17 9 42
Removing an element from a Doubly Linked List has the same steps
as inserting an element to the list, just in reverse order.
2015, Marcus Biel, http://www.marcus-biel.com/
public class Node<E> { private E item; private Node<E> next; public Node(E element, Node<E> next) { this.item = element; this.next = next; }
public E item() { return item;}
public Node<E> 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.
2015, Marcus Biel, http://www.marcus-biel.com/
Nodepublic class Node<E> { private E item; private Node<E> previous; private Node<E> next;
public Node(Node<E> previous, E element, Node<E> next) { this.item = element; this.next = next; this.previous = previous; }
public E item() { return item;}public Node<E> previous() { return previous;}public Node<E> 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.
2015, Marcus Biel, http://www.marcus-biel.com/
public class LinkedList<E> { private Node<E> currentNode; private Node<E> 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.
2015, Marcus Biel, http://www.marcus-biel.com/
public class LinkedList<E> { private Node<E> currentNode; private Node<E> 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.
2015, Marcus Biel, http://www.marcus-biel.com/
public class LinkedList<E> { private Node<E> currentNode; private Node<E> head; private Node<E> 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.
2015, Marcus Biel, http://www.marcus-biel.com/
Application ScenariosListQueueStackDouble 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.
2015, Marcus Biel, http://www.marcus-biel.com/
Application ScenariosListQueueStackDouble Ended Queue (Deque)
When comparing implementations based on a Singly- or a Doubly Linked List,
there is no overall winner.
2015, Marcus Biel, http://www.marcus-biel.com/
Application ScenariosListQueueStackDouble Ended Queue (Deque)
Both have advantages and disadvantages that make them useful for different application scenarios.
2015, Marcus Biel, http://www.marcus-biel.com/
Application ScenariosListQueueStackDouble 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.
2015, Marcus Biel, http://www.marcus-biel.com/
Application ScenariosListQueueStackDouble Ended Queue (Deque)
An implementation with other data structures, for example an array,
is also possible, but that’s a different story.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble 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.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble 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.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble Ended Queue (Deque)
Application Scenarios
Random access is not required, so I would recommend
an implementation based on a Singly Linked List.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble 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.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble Ended Queue (Deque)
Application Scenarios
Therefore a Singly Linked List is usually sufficient.
2015, Marcus Biel, http://www.marcus-biel.com/
ListQueueStackDouble 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.