Data Structure Marcus Biel Software Craftsman httpwwwmarcusbielcom Terminology First of all lets have a look at the term Linked List Why is Linked List actually called Linked List ID: 503922
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Slide1
Linked ListData Structure
Marcus Biel, Software
Craftsman
http://www.marcus-biel.comSlide2
Terminology
First of all, let’s have a look at the term “Linked List”.
Why
is Linked List actually called Linked List?Slide3
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.Slide4
Terminology
List
A list is a collection of related objects.
Think
of a shopping list - it contains every item that you plan to buy.Slide5
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
.Slide6
Terminology
Linked List
Technically, in our Linked List, an item or entry of the list…Slide7
Node
… is stored in a so called “Node
”.Slide8
23
Node
In our simple example, this entry is the number twenty three
.Slide9
Node
23
Each node has a link or pointer to the next element of the list,
the
next node
.Slide10
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
.Slide11
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
.Slide12
Singly Linked List
23
3
For every new item, we create a new node
…Slide13
Singly Linked List
23
9
3
… and link to it from the last element of the list
.Slide14
Singly Linked List
23
9
3
42
So a Linked List is a dynamic data
structure,
that
can hold any number of elements,
as
long as enough memory is available
.Slide15
Singly Linked List
next ( )
23
9
3
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. Slide16
Singly Linked List
23
9
3
42
This type of a Linked List is called a Singly Linked List
because
the links only go in one direction
-Slide17
Singly Linked List
23
9
3
42
from the head of the list - Slide18
Singly Linked List
23
9
3
42
- to the tail. Slide19
Singly Linked List
23
9
3
42
So to navigate to the previous element,
for
example from the element 42 - Slide20
Singly Linked List
23
9
3
42
to the element
9Slide21
Singly Linked List
23
9
3
42
you
have to go back to the head of the list
-Slide22
Singly Linked List
next ( )
23
9
3
42
and you have to call the function next() -Slide23
Singly Linked List
23
9
3
42
on every single element
-Slide24
Singly Linked List
next ( )
23
9
3
42
until you reach the element 9
.Slide25
Singly Linked List
23
9
3
42
If the list contains a lot of elements, this may take some time
.Slide26
Singly Linked List
23
9
3
42
Inserting an element after the
current
element is relatively easy
with
a Singly Linked List. Slide27
Singly Linked List
17
23
9
3
42
You create a new Node containing the new element
.Slide28
Singly Linked List
17
23
9
3
42
You link to the new element „17“ from the current element „9
”.Slide29
Singly Linked List
17
23
9
3
42
You add a link pointing from the new “17” element
to
the existing “42” element
.Slide30
Singly Linked List
23
9
3
17
42
And
you’r
done
.Slide31
Singly Linked List
23
9
3
42
Inserting the same element BEFORE
the
current element
is
possible in a Singly Linked List,
but
usually not very efficient
.Slide32
Singly Linked List
23
9
3
42
It will require to navigate to the previous element,
starting
from the beginning of the list, as I showed you before
.Slide33
Singly Linked List
23
9
3
42
Removing an element from a Singly Linked List
has
the same issue
–
it is possible, but generally not very efficient.Slide34
Node
23
These operations get much
easier,
when you add a second link to each node,
pointing
to the previous element. Slide35
Node
23
This way you can navigate in both directions of the list
.Slide36
Node
23
However the extra link comes at a cost, of extra memory,
as
well as time to build the more complex structure
.Slide37
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. Slide38
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
”.Slide39
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. Slide40
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
.Slide41
Doubly Linked List
23
3
9
42
Based on the two way link structure
…Slide42
Doubly Linked List
23
3
9
42
17
adding or removing an element before
or
after
the current element is relatively easy
.Slide43
Doubly Linked List
23
3
9
42
17
Here we will add the element “17” before the current element “9
”.Slide44
Doubly Linked List
23
3
9
42
17
The back link from element “9” to element “3” is removed
…Slide45
Doubly Linked List
23
3
9
42
17
and replaced by a back link to the new element “17
”.Slide46
Doubly Linked List
23
3
9
42
17
From the new element “17” we link to the next element “9
”.Slide47
Doubly Linked List
23
3
9
42
17
…next, we place a back link from 17 to 3
…Slide48
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
.Slide49
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. Slide50
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. Slide51
Node
public 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
.Slide52
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
.Slide53
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. Slide54
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
.Slide55
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
.Slide56
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
.Slide57
Application Scenarios
List
Queue
Stack
Double Ended Queue (
Deque
)
Both have advantages and disadvantages that
make
them useful for different application scenarios
.Slide58
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
.Slide59
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. Slide60
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
.Slide61
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
.Slide62
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
.Slide63
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
.Slide64
List
Queue
Stack
Double Ended Queue (
Deque
)
Application Scenarios
Therefore a Singly Linked List is usually sufficient
.Slide65
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
.