Basic ideas about linked list

1. Linked Lists

2. 2
Anatomy of a linked list
 A linked list consists of:
 A sequence of nodes
a b c d
 Each node contains a value
 and a link (pointer or reference) to some other node
 The last node contains a null link
 The list may (or may not) have a header
myList
 myList isn’t a header, it’s just a reference

3. 3
More terminology
 A node’s successor is the next node in the sequence
 The last node has no successor
 A node’s predecessor is the previous node in the
sequence
 The first node has no predecessor
 A list’s length is the number of elements in it
 A list may be empty (contain no elements)

4. 4
Creating links
typedef struct link
{
int data;
struct link *next;
}myList;
myList *head, *tail;
int main()
{
myList *p, *q;
p=(myList*)malloc(sizeof(myList));
q=(myList*)malloc(sizeof(myList));
p->data=44;
q->data=97;
p->next=q;
q->next=0;
 }
data next pointer
data next
data next
p:
q:
44
97
next
next
p:
q:
44 97
p: q:

5. 5
Singly-linked lists
 Here is a singly-linked list (SLL):
 Each node contains a value and a link to its successor
(the last node has no successor)
 We have a reference to the first node in the list
 The reference is null if the list is empty
a b c d
myList

6. 6
Creating a simple list
 A head node is just an initial node that exists at the front of
every list, even when the list is empty
 The purpose is to keep the list from being null, and to point at
the first element
 A tail node is just an initial node that exists at the end of every
list, even when the list is empty.
 The purpose is to keep the list from being null, and to point at
the last element
 In the previous code, just add this in main:
head=p
tail=q
44 97
p: q:
head:
tail:

7. 7
Traversing a SLL
 The following method traverses a list (and
prints its elements):
void traverse()
{ p=head;
while(p!=0)
{
printf(“%d “, p->data);
p=p->next;
}

8. 8
Traversing a SLL (animation)
489744
head
p

9. 9
Inserting a node into a SLL
 There are many ways you might want to insert a new
node into a list:
 As the new first element
 As the new last element
 Before a given value
 After a given value
 All are possible, but differ in difficulty

10. 10
Inserting as a new last element
 This is probably the easiest method to implement
 void insertAtLast(int value)
{
p=(myList*)malloc(sizeof(myList));
p->data=value;
p->next=tail->next;
tail=p;
}
 Use this as: insertAtLast(value);

11. 11
Inserting at last (animation)
9744
head
48p
Find the tail node
First, copy the link from the node that’s already in the list
Then, change the link in the node that’s already in the list
tail
Then, change the link in the tail node
Example: Call: insertAtLast(48)

12. 12
Inserting a node as a new first element
 void insertAtFirst(int value)
{
p=(myList*)malloc(sizeof(myList));
p->data=value;
p->next=head;
head=p;
}
 Use this as: insertAtFirst(value)

13. 13
Inserting at first (animation)
9744
head
14p
Find the head node
First, change the link in the new node
tail
Then, change the link in the head node
48
Example: Call: insertAtFirst(14)

14. 14
Inserting a node as a new first element
 void insertAtMiddle(int search_data, int value)
{
q=head;
while(q!=0)
{
if(q->data==search_data)
{
p=(myList*)malloc(sizeof(myList));
p->data=value;
p->next=q->next;
q->next=p;
}
p=p->next;
}
Use this as: insertAtMiddle(search_data, value)
Don’t change
the sequence

15. 15
Inserting after (animation)
489744
head
29p
Find the node you want to insert after
First, copy the link from the node that’s already in the list
Then, change the link in the node that’s already in the list
q
Example: Call: insertAtMiddle(29)

16. 16
Deleting a node from a SLL
 In order to delete a node from a SLL, you have to
change the link in its predecessor
 This is slightly tricky, because you can’t follow a
pointer backwards
 Deleting the first node in a list is a special case, because
the node’s predecessor is the list header

17. 17
Inserting as a new last element
 This is probably the easiest method to implement
 void delete(int value)
{ q=0;
p=head;
while(p!=0)
{
if(p->data==value)
{
if(p==head)
head=p->next;
else
q->next=p->next;
}
q=p;
p=p->next;
}
Use this as: delete(value);

18. 18
Deleting an element from a SLL
489744
head
• To delete the first element, change the link in the header
• To delete some other element, change the link in its predecessor
• What will happen for last node??? Code &Visualize yourself
(predecessor)
974414
head
48
q p
p

19. 19
Other operations on linked lists
 Most “algorithms” on linked lists—such as insertion,
deletion, and searching—are pretty obvious; you just
need to be careful
 Sorting a linked list is just messy, since you can’t
directly access the nth
element—you have to count your
way through a lot of other elements
Question: How would you reverse a singly-linked list in
place (that is, without creating any new nodes)?

20. 20
The End