Created by Zaheer Abbas Aghani

Data Structure & AlgorithmsITC-306

Lecture 11 &12

REPRESENTATION OF LINKED LIST IN MEMORY

A P P L E

START

1

2

3

4

5

6

7

8

START

6

A

P

P

L

E

2

1

7

4

0

INFO

LINK

INVALID ADDRESS

LINKED LIST ENDS

Linked List Operations:

The operation normally performed on linked list are:

Traversing: Process each element in the list. Searching: Finding the location of an element. Insertion: Insert new elements/ nodes in to list. Deletion: Delete an elements/ nodes from list.

Traversing Operation

Suppose we want traverse list in order to process each node exactly once.

Now here we write an algorithm that shows how to perform traversing operation on linked list.

Let LIST be a linked list in memory. In linked list start pointing to the first element & null indicating the end of list.

Our traversing algorithm uses a pointer variable PTR which points to the node currently being processed.

Accordingly LINK[PTR] points to the next node to be processed. Thus the assignment

PTR:=LINK[PTR]

Moves the pointer to the next node in the list.

Traversing algorithm

The following is an algorithm that perform traversing operation on linked list. This purpose of this algorithm is to print each element of list. The variable PTR points to the node currently being processed.

Step1: Start. Step2: set PTR:=START [initialize pointer PTR] Step3: Repeat step4 and 5 while PTR!=NULL. Step4: Print INFO[PTR]. Step5: set PTR:=LINK[PTR]. (PTR now points to the next node) End of step3 loop. Step6: Exit.

Searching into a linked list:

Searching refers to search an element in a linked list. For searching the element we first traverse the linked list and with traversing we compare the INFO part of each element with the given element. The algorithm for searching operation is written as:

Step1: Start. Step2: Set PTR to START [initialize pointer PTR] Step3: Repeat step4 while PTR!=NULL. Step4: if INFO[PTR]==ITEM then

print element foundElse:

Set PTR:=LINK[PTR]. (PTR now points to the next node)(End of step3 loop)

Step5: Exit.

Insertion into a linked list:

Insertion into linked list may be possible in two ways:

1.Insertion at beginning 2.Insertion in between.

Insertion Algorithm

CASE 1 (insertion at beginning)

Suppose temp is node that has to be inserted.

In linked list START points to the first element of linked list.

Step1: Start Step2: For insertion at beginning

we assign the value of START to the link part of inserted node.

Now inserted node points to the next node which was beginning node of list as Temp[PTR]=START.

Step3: assign the address of inserted node to the start pointer variable as START=temp.

Now START will point to the inserted node(temp) which is first node of linked list.

Step4: exit

Start

Null

temp

10

10 70 40

70 40

42

Start Null

temp

40

70

70

4010

10

42

Start Null

temp

40

70

70

4010

10

42

42

CASE 2 (insertion in between)

In case 2 first we traverse the list for obtaining the node after which we want to insert the element.

Here we have a linked list with 3 nodes (A, C & D)

Now here suppose we want to insert the node B in b/w A and C node. Then

Step1: First we assign the link part of node A to link part of node B as B[PTR]=A[PTR]

Now node B points to the node C Step2: Secondly assign the

address of node B to the link part of node A as A[PTR]=B.

Now node A points to node B and node B points to node C.

Step3: exit

start A C D

start A C D

B

start A C D

B

10 42 30

42 3010

10

10 42

42

30

30

22

2210

10 42

42

30

30

Deletion from a linked list:

For deleting the node from a linked list, first we traverse the linked list and compare with each element. After finding the element there are may be two cases for deletion.

1. Deletion at beginning 2. Deletion in between.

Deletion Algorithm: Case 1 (deletion at

beginning) Suppose temp is node that

has to be deleted. If the element to be deleted is the first node of linked list then first

Step1: assign the linked part of deleted node (temp) to the start.

After that start points to the second node of list and now we should free the temp node to be deleted

Step2: delete node temp. Step3: exit.

tempstart

A B C

21

21

55

55

42

42

Null

Nulltempstar

t

A B C21

55

55

55 4

2

42

tempstart Null

55

55

42

42

A B C

Deletion Algorithm:

Case 1 (deletion at beginning)

Step1: Start Step2: Set PTR=START Step3: Assign START=LINK[PTR]

now Start points out the second node in list.Step4: Delete first Node.(A node)Step5: Exit/End

Case 2 deletion in between In case 2 first we traverse the

list for obtaining the node which we want to delete.

Now here suppose we want to delete node B which is in b/w A and C node. Then

Step1: assign the link part of deleted node (node B) to the pervious node (node A)

After step1 node A is pointed to the node C.

Now we should delete the node B.

Step2: delete node B.

Step3: exit.

start A B C

20

20 4

2

42

111

111

start A B C

111

1112

0

20 4

2

start A B C20 2

042

111 1

11

Case 2 deletion in between

Step1: Start Step2: set PTR=START (PTR points to 1st node)

Step3: set PTR1=LINK[PTR] (LINK[PTR] points to 2nd node)

Step4: Assign LINK[PTR]=LINK[PTR1] or A[PTR]=B[PTR]

Step5: Delete Node. Step6: Exit/End

Deletion at last. If node to be deleted is the last node of list

then first we assign null value to the link part of last node to pervious node then secondly delete the last node.

Step 1:

Step 2:

start A B C

start A B C

start A B C

Deletion at last.

Step1: Start Step2: set PTR=START Step3: set PTR1=LINK[PTR] Step4: Assign LINK[PTR1]=

NULL Step5: Delete last Node Step6: Exit/End