1. Linked Lists
Damian Gordon

2. Linked Lists
Imagine we wanted to have an array where we can dynamically add elements into it, and take them away.
We can do this with a linked list.

3. Linked Lists
A linked list is made up of nodes.

4. Linked Lists A linked list is made up of nodes.
Each node has two parts to it:

5. Linked Lists A linked list is made up of nodes.
Each node has two parts to it:
Value
Pointer

6. Linked Lists
For example

7. Linked Lists
For example 23

8. Linked Lists
For example 23 62

9. Linked Lists
For example 23 62 37

10. Linked Lists
For example 23 62 37 31

11. Linked Lists
For example
NULL 23 62 37 31

12. Linked Lists
For example
NULL 23 62 37 31
Start
of List
End
of List

13. Linked Lists
To add a value in:
NULL 23 62 37 31

14. Linked Lists
To add a value in:
NULL 23 62 37 31 26

15. Linked Lists
To add a value in:
NULL 23 62 37 31 26

16. Linked Lists
To add a value in:
NULL 23 62 37 31 26

17. Linked Lists
To add a value in:
NULL 23 62 26 37 31

18. Linked Lists
To delete a value:
NULL 23 62 26 37 31

19. Linked Lists
To delete a value:
NULL 23 62 26 37 31

20. Linked Lists
To delete a value:
NULL 23 62 37 31

21. Linked Lists So a Linked List is a series of Nodes
And a Node has two parts
Value
Pointer
A Linked List has a pointer to the start of the list called Head.

22. Linked Lists
To add a value in:
Value
Pointer
NULL 23 62 26 37 31
HEAD

23. Linked Lists
TYPE Node: INTEGER Value; NODE Pointer; ENDTYPE;

24. Linked Lists We will look at implementing the following modules:
CreateList()
Create an empty linked list
DeleteList()
Delete a linked list
IsEmpty()
Check if the linked list is empty

25. CreateList()

26. Linked Lists
To create a list:
NULL
HEAD

27. Linked Lists
MODULE CreateList(): Head <- NULL; END.

28. DeleteList()

29. Linked Lists
To delete a list:
Value
Pointer
NULL 23 62 26 37 31
HEAD

30. Linked Lists
To delete a list:
Value
Pointer
NULL 23 62 26 37 31
HEAD

31. Linked Lists
MODULE DeleteList(): Head <- NULL; END.

32. IsEmpty()

33. Linked Lists
To check if a linked list is empty:
NULL
HEAD

34. Linked Lists
MODULE IsEmpty(): Boolean Empty; IF Head = NULL THEN Empty <- True; ELSE Empty <- False; ENDIF; RETURN Empty; END.

35. Linked Lists Or
MODULE IsEmpty():
RETURN Head = NULL;
END.

36. Linked Lists We will look at implementing the following modules:
DisplayList()
print out all the nodes
FindNode(N)
Find a node with a given value (N)
InsertNode(Pos, N)
Insert a node at a particular position
DeleteNode(N)
Delete a node with a given value (N)

37. DisplayList()

38. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

39. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

40. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

41. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

42. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

43. NULL Current HEAD Linked Lists
To display all the values, start at the Head, and end at NULL.
NULL 23 62 26 37 31
Current
HEAD

44. Linked Lists
MODULE DisplayList(): Node Current <- Head; WHILE (Current.Pointer != NULL) DO Print Current.Value; Current <- Currrent.Pointer; ENDWHILE; END.

45. Linked Lists
To print out a count of the number of nodes:

46. Linked Lists
MODULE DisplayList(): Node Current <- Head; Integer CountNodes <- 0; WHILE (Current.Pointer != NULL) DO Print Current.Value; Current <- Currrent.Pointer; CountNodes <- CountNodes + 1; ENDWHILE; Print “Number of Nodes:”, CountNodes; END.

47. Linked Lists
MODULE DisplayList(): Node Current <- Head; Integer CountNodes <- 0; WHILE (Current.Pointer != NULL) DO Print Current.Value; Current <- Currrent.Pointer; CountNodes <- CountNodes + 1; ENDWHILE; Print “Number of Nodes:”, CountNodes; END.

48. FindNode(N)

49. NULL Current HEAD Linked Lists To find a value (e.g. 37) in the list:23 62 26 37 31
Current
HEAD

50. NULL Current HEAD Linked Lists To find a value (e.g. 37) in the list:23 62 26 37 31
Current
HEAD

51. NULL Current HEAD Linked Lists To find a value (e.g. 37) in the list:23 62 26 37 31
Current
HEAD

52. NULL Current HEAD Linked Lists To find a value (e.g. 37) in the list:23 62 26 37 31
Current
HEAD

53. Linked Lists
MODULE FindNode(N): Node Current <- Head; WHILE (Current.Value != N) AND (Current.Pointer != NULL) DO Current <- Current.Pointer; ENDWHILE; IF Current.Pointer = NULL THEN Print “Value not found”; ELSE Print “Value found”; ENDIF; END.
HEAD
Current

54. InsertNode(Pos, N)

55. NULL Current HEAD Linked Lists
To add in a value after position 4, of value N:
NULL 23 62 26 37 31
Current
HEAD

56. NULL Current HEAD Linked Lists
To add in a value after position 4, of value N:
NULL 23 62 26 37 31
Current
HEAD

57. NULL Current HEAD Linked Lists
To add in a value after position 4, of value N:
NULL 23 62 26 37 31
Current
HEAD

58. NULL Current HEAD Linked Lists
To add in a value after position 4, of value N:
NULL 23 62 26 37 31
Current
HEAD

59. NULL Current HEAD Linked Lists N
To add in a value after position 4, of value N:
NULL 23 62 26 37 31 N
Current
HEAD

60. NULL Current HEAD Linked Lists N
To add in a value after position 4, of value N:
NULL 23 62 26 37 31 N
Current
HEAD

61. NULL Current HEAD Linked Lists N
To add in a value after position 4, of value N:
NULL 23 62 26 37 31 N
Current
HEAD

62. Linked Lists
First declare the important variables, and then create a new node:
MODULE InsertNode(Pos, N):
Node Current <- Head;
Integer PositionCounter;
PositionCounter = 1;
Node NewNode;
NewNode.Value <- N; N

63. Linked Lists Next let’s find the position in the linked list:
WHILE (Pos > PositionCounter)
DO Current <- Current.Pointer;
PositionCounter <- PositionCounter + 1;
ENDWHILE;

64. Linked Lists
Now add to the start if it’s Pos 0, otherwise add it in after the position pointed to by Current.
IF Pos = 0
THEN NewNode.Pointer <- Head;
Head <- NewNode;
ELSE NewNode.Pointer <- Current.Pointer;
Current.Pointer <- NewNode;
ENDIF;
NewNode
HEAD
Current

65. Linked Lists
MODULE InsertNode(Pos, N): Node Current <- Head; Integer PositionCounter; Node NewNode; NewNode.Value <- N; WHILE (Pos > PositionCounter) DO Current <- Current.Pointer; PositionCounter <- PositionCounter + 1; ENDWHILE; IF Pos = 0 THEN NewNode.Pointer <- Head; Head <- NewNode; ELSE NewNode.Pointer <- Current.Pointer; Current.Pointer <- NewNode; ENDIF; END.

66. DeleteNode(N)

67. NULL Previous Current HEAD
Linked Lists
To delete a value (e.g. 37) in the list:
NULL 23 62 26 37 31
Previous
Current
HEAD

68. NULL Previous Current HEAD
Linked Lists
To delete a value (e.g. 37) in the list:
NULL 23 62 26 37 31
Previous
Current
HEAD

69. NULL Previous Current HEAD
Linked Lists
To delete a value (e.g. 37) in the list:
NULL 23 62 26 37 31
Previous
Current
HEAD

70. NULL Previous Current HEAD
Linked Lists
To delete a value (e.g. 37) in the list:
NULL 23 62 26 37 31
Previous
Current
HEAD

71. NULL Previous Current HEAD
Linked Lists
To delete a value (e.g. 37) in the list:
NULL 23 62 26 37 31
Previous
Current
HEAD

72. Linked Lists
Check if the required value is in the first node, if so, move the HEAD to the next node, and then it’s deleted:
MODULE DeleteNode(N):
Node Previous <- NULL;
Node Current <- Head;
IF Current.Value = N
THEN Head <- Current.Pointer;
ENDIF;
END.
HEAD

73. Linked Lists
Otherwise find the node in the same way as the previous function:
WHILE (Current.Value != N) AND
(Current.Pointer != NULL)
DO Previous <- Current;
Current <- Current.Pointer;
ENDWHILE;
HEAD
Previous
Current

74. Linked Lists And then delete it: IF (Current.Value = N)
THEN Previous.Pointer <- Current.Pointer;
ELSE PRINT “Not found”
ENDIF;
HEAD
Previous
Current

75. Linked Lists
MODULE DeleteNode(N): Node Previous <- NULL; Node Current <- Head; IF Current.Value = N THEN Head <- Current.Pointer; ELSE WHILE (Current.Value != N) AND (Current.Pointer != NULL) DO Previous <- Current; Current <- Current.Pointer; ENDWHILE; IF (Current.Value = N) THEN Previous.Pointer <- Current.Pointer; ELSE PRINT “Not found” ENDIF; END.

76. Linked Lists
To print out a count of the node number where the delete was:

77. Linked Lists
MODULE DeleteNode(N): Node Previous <- NULL; Node Current <- Head; Integer PositionCounter; IF Current.Value = N THEN Head <- Current.Pointer; ELSE WHILE (Current.Value != N) AND (Current.Pointer != NULL) DO Previous <- Current; Current <- Current.Pointer; PositionCounter <- PositionCounter + 1; ENDWHILE; IF (Current.Value == N) THEN Previous.Pointer <- Current.Pointer; Print “Node deleted at position:”, PositionCounter; ELSE PRINT “Not found” ENDIF; END.

78. Linked Lists
MODULE DeleteNode(N): Node Previous <- NULL; Node Current <- Head; Integer PositionCounter; IF Current.Value = N THEN Head <- Current.Pointer; ELSE WHILE (Current.Value != N) AND (Current.Pointer != NULL) DO Previous <- Current; Current <- Current.Pointer; PositionCounter <- PositionCounter + 1; ENDWHILE; IF (Current.Value == N) THEN Previous.Pointer <- Current.Pointer; Print “Node deleted at position:”, PositionCounter; ELSE PRINT “Not found” ENDIF; END.

79. etc.