Lec 7 Double Linked List

8/10/2019 Lec 7 Double Linked List

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Lecture : 7

Doubly Linked List

Instructor: Anum Masood

1


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Drawback of Single Linked List

In single linked list we can traverse only in onedirection because each node has address of nextnode only.

Suppose we are in middle of linked list and wewant to perform operation with just previousnode then we have no way to go back onprevious node, we will again traverse fromstarting node.

Above is the main drawback of single linked list.


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Solution of the problem: Doubly

Linked List To solve the problem of single linked list a data structure

named doubly linked list is introduced.

In doubly linked list each node has address of both nodes

which are previous and next node.

The data structure for doubly linked list will be as:

struct node{

struct node *previous;int info;

struct node *next;

}


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Example: For Understanding Doubly

Linked List

start

start

next

NULLPrev

NULL


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Doubly Linked List

start


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Doubly Linked List

Here struct node *prev is a pointer to structure, which

will contain the address of previous node. Struct node *next contains the address of next node in

the list.

So in doubly linked list traversing can be performed in

both direction.

The only drawback is that each node has to contain the

address information of previous node too.

start


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Traversing a Doubly Linked List

For processing the next element assign the address ofnext node to ptr as:

ptr=ptr->next;

Now ptr has the address of next node.

Traverse each element of list through this assignmentuntil ptr has NULL value which is the next part value oflast element. So the doubly lined list can be traversed

as:

while(ptr!=NULL)

ptr=ptr->next;


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Insertion into a Doubly Linked List

Insertion in a doubly linked list may be

possible in two ways:

1. Insertion at beginning

2. Insertion in between


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Case 1: Insertion at beginning

Start points to the first node of doubly linked list.

For insertion at beginning, assign the value of start tothe next part of the inserted node and address of the

inserted node to the prev part of start as:temp->next=start;

start->prev=temp;

Now inserted node points to the next node, which wasbeginning node of the doubly linked list and prev partof second node will point to the new inserted node.


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Case 1: Insertion at beginning

Now inserted node is the first node of the doublylinked list, so start will be reassigned as:

start=temp;

Now start will point to the inserted node which isthe first node of the doubly linked list.

Assign NULL to prev part of inserted node sincenow it will become the first node and prev part offirst node is NULL

temp->prev=NULL;


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Case 2: Insertion in between

First traverse the doubly linked list for obtaining thenode after which new element is to be inserted.

For inserting the element after the node assign the

address of inserted node to the prev part of the nextnode.

Then assign the next part of previous node to the next

part of inserted nose.

Address of previous node will be assigned to the prevpart of inserted node and address of inserted node willbe assigned to next part of previous node.


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q


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Step: 1

q->next->prev=temp;

q


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Step : 2

temp->next=q->next;q


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Step 3:

temp->prev=q;

q


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Step 4:

q->next=temp;

q


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After statement 3, prev part of inserted nose

will point to its previous node.

After statement 4, next part of previous node

will point to inserted node.


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DELETION


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Deletion from Doubly Linked List

Insertion in a doubly linked list may be

possible in two ways:

1. Deletion at beginning

2. Deletion in between

3. Deletion of last node


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Case 1: Deletion at beginning

Start points to the first node of doubly linked list.

If first node contains the key i.e. num; then assign:

temp = start;

start = start->next; // 2nd node into start

start->prev= NULL;

free(temp)

Start points to the first node and start-> next pointsto 2nd node.

Then NULL will be assigned to the start->prev

Afterwards delete temp.


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Case 2: Deletion in between

If the element is other than the first nodethen first traverse the doubly linked list forobtaining the node (containing key).

Assign next part of deleted node to the nextpart of the previous node.

Address of previous node will be assigned tothe prev part of next node.


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1. temp = q->next;2. q->next = temp->next;3. temp->next->prev = q;4. free(temp);

Here q point to the previous node of the deletednode.

After statement 1, temp contains the address of thenode to be deleted.

Statement 2, next part of the previous node will pointto the next node of the node to be deleted

Statement 3, prev part of the next node will point tothe previous node.


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q

q

q

q

temp

temp

temp


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start

q


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Case 3: Deletion from End

If the node to be deleted is last node of doubly linkedlist then next part of the 2nd last node will containNULL.

1. temp = q->next; // last node2. free(temp)

3. q->next= NULL;

q points to the 2nd last node and q-> next points tolast node.

Then NULL will be assigned to the start->prev

Afterwards delete temp.


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Questions

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Documents

Lec 7 Double Linked List