Linked lists in Data Structure

Preliminaries
• Options for implementing an ADT List
– Array has a fixed size
• Data must be shifted during insertions and deletions
– Linked list is able to grow in size as needed

Introduction to the Linked List ADT
• A linked list is a series of connected nodes,
where each node is a data structure.
• A linked list can grow or shrink in size as the
program runs

Advantages of Linked Lists over Arrays
and vectors
• A linked list can easily grow or shrink in size.
• Insertion and deletion of nodes is quicker with
linked lists than with vectors.
4

The composition of a Linked List
• Each node in a linked list contains one or
more members that represent data.
• In addition to the data, each node contains
a pointer, which can point to another node.
5

The composition of a Linked List
• A linked list is called "linked" because each
node in the series has a pointer that points
to the next node in the list.
6

Declarations
• First you must declare a data structure that
will be used for the nodes. For example, the
following struct could be used to create
a list where each node holds a float:
7
struct ListNode
{
float value;
struct ListNode *next;
};

Declarations
• The next step is to declare a pointer to
serve as the list head, as shown below.
8
ListNode *head;
• Once you have declared a node data
structure and have created a NULL head
pointer, you have an empty linked list.
• The next step is to implement operations
with the list.

Linked List Operations
• We will use the following class declaration (on the
next slide), which is stored in FloatList.h.
9

10
class FloatList
{
private:
// Declare a structure for the list
struct ListNode
{
float value;
};
ListNode *head; // List head pointer
public:
FloatList(void) // Constructor
{ head = NULL; }
~FloatList(void); // Destructor
void appendNode(float);
void insertNode(float);
void deleteNode(float);
void displayList(void);
};

Appending a Node to the List
• To append a node to a linked list means to add the node to
the end of the list.
• The pseudocode is shown below. The C++ code follows.
11
Create a new node.
Store data in the new node.
If there are no nodes in the list
Make the new node the first node.
Else
Traverse the List to Find the last node.
Add the new node to the end of the list.
End If.

12
void FloatList::appendNode(float num)
{
ListNode *newNode, *nodePtr;
// Allocate a new node & store num
newNode = new ListNode;
newNode->value = num;
newNode->next = NULL;
// If there are no nodes in the list
// make newNode the first node
if (!head)
head = newNode;
else // Otherwise, insert newNode at end
{
// Initialize nodePtr to head of list
nodePtr = head;
// Find the last node in the list
while (nodePtr->next)
nodePtr = nodePtr->next;
// Insert newNode as the last node
nodePtr->next = newNode;
}
}

Program 1
13
// This program demonstrates a simple append
// operation on a linked list.
#include <iostream.h>
#include "FloatList.h”
void main(void)
{
FloatList List;
list.appendNode(2.5);
}
(This program displays no output.)

Stepping Through the Program
• The head pointer is declared as a global variable.
head is automatically initialized to 0 (NULL),
which indicates that the list is empty.
• The first call to appendNode passes 2.5 as the
argument. In the following statements, a new
node is allocated in memory, 2.5 is copied into its
value member, and NULL is assigned to the
node's next pointer.
14

15
newNode->next = nULL;

16
The next statement to execute is the following if statement.
if (!head)
head = newNode;
There are no more statements to execute, so control returns to function main.

17
In the second call to appendNode, 7.9 is passed as the argument. Once again, the first
three statements in the function create a new node, store the argument in the node's
value member, and assign its next pointer to NULL.

18
Since head no longer points to NULL, the else part of the if statement executes:
{
nodePtr = head;
}

19
nodePtr is already at the end of the list, so the while loop immediately terminates.
The last statement, nodePtr->next = newNode; causes nodePtr->next to point
to the new node. This inserts newNode at the end of the list.

20
The third time appendNode is called, 12.6 is passed as the argument. Once again, the
first three statements create a node with the argument stored in the value member.

21
next, the else part of the if statement executes. As before, nodePtr is made to
point to the same node as head.

22
Since nodePtr->next is not NULL, the while loop will execute. After its first
iteration, nodePtr will point to the second node in the list.

23
The while loop's conditional test will fail after the first iteration because nodePtr-
>next now points to NULL. The last statement, nodePtr->next = newNode;
causes
nodePtr->next to point to the new node. This inserts newNode at the end of the list
The figure above depicts the final state of the linked list.

Traversing the List
• The displayList member function traverses the list,
displaying the value member of each node. The following
pseudocode represents the algorithm. The C++ code for the
member function follows on the next slide.
24
Assign List head to node pointer.
While node pointer is not NULL
Display the value member of the node pointed to by node pointer.
Assign node pointer to its own next member.
End While.

25
void FloatList::displayList(void)
{
ListNode *nodePtr;
nodePtr = head;
while (nodePtr)
{
cout << nodePtr->value << endl;
}
}

Program 2
26
// This program calls the displayList member function.
// The funcion traverses the linked list displaying
// the value stored in each node.
#include "FloatList.h"
void main(void)
{
FloatList List;
list.displayList();
}

Program 2 Output
27
2.5
7.9
12.6

Inserting a Node
• Using the listNode structure again, the
pseudocode on the next slide shows an
algorithm for finding a new node’s proper
position in the list and inserting there.
• The algorithm assumes the nodes in the list
are already in order.
28

29
Create a new node.
Store data in the new node.
If there are no nodes in the list
Make the new node the first node.
Else
Find the first node whose value is greater than or equal
the new value, or the end of the list (whichever is first).
Insert the new node before the found node, or at the end of
the list if no node was found.
End If.

30
The code for the traversal algorithm is shown below. (As before, num holds the value being
inserted into the list.)
nodePtr = head;
// Skip all nodes whose value member is less
// than num.
while (nodePtr != NULL && nodePtr->value < num)
{
previousNode = nodePtr;
}
The entire insertNode function begins on the next slide.

31
void FloatList::insertNode(float num)
{
ListNode *newNode, *nodePtr, *previousNode;
// Allocate a new node & store Num
if (!head)
{
head = newNode;
}
else // Otherwise, insert newNode.
{
nodePtr = head;
// than num.
{
} Continued on next slide…

32
// If the new mode is to be the 1st in the list,
// insert it before all other nodes.
if (previousNode == NULL)
{
head = newNode;
newNode-> = nodePtr;
}
else
{
previousNode->next = newNode;
newNode->next = nodePtr;
}
}
}
Continued from previous slide.

Program 3
33
// This program calls the displayList member function.
// The function traverses the linked list displaying
// the value stored in each node.
#include "FloatList.h”
void main(void)
{
FloatList list;
// Build the list
// Insert a node in the middle
// of the list.
list.insertNode(10.5);
// Dispay the list
list.displayList();
}

Program 3 Output
34
2.5
7.9
10.5
12.6

35
In insertNode, a new node is created and the function argument is copied to its value
member. Since the list already has nodes stored in it, the else part of the if statement
will execute. It begins by assigning nodePtr to head.

36
Since nodePtr is not NULL and nodePtr->value is less than num, the while loop will
iterate. During the iteration, previousNode will be made to point to the node that
nodePtr is pointing to. nodePtr will then be advanced to point to the next node.

37
Once again, the loop performs its test. Since nodePtr is not NULL and nodePtr-
>value is less than num, the loop will iterate a second time. During the second iteration,
both previousNode and nodePtr are advanced by one node in the list.

38
This time, the loop's test will fail because nodePtr is not less than num. The statements
after the loop will execute, which cause previousNode->next to point to newNode,
and
newNode->next to point to nodePtr.
If you follow the links, from the head pointer to the NULL, you will see that the nodes are
stored in the order of their value members.

Deleting a Node
• Deleting a node from a linked list requires two
steps:
– Remove the node from the list without breaking
the links created by the next pointers
– Deleting the node from memory
• The deleteNode function begins on the
next slide.
39

40
void FloatList::deleteNode(float num)
{
ListNode *nodePtr, *previousNode;
// If the list is empty, do nothing.
if (!head)
return;
// Determine if the first node is the one.
if (head->value == num)
{
nodePtr = head->next;
delete head;
head = nodePtr;
}
Continued on next slide…

41
else
{
nodePtr = head;
// Skip all nodes whose value member is
// not equal to num.
while (nodePtr != NULL && nodePtr->value != num)
{
}
// Link the previous node to the node after
// nodePtr, then delete nodePtr.
previousNode->next = nodePtr->next;
delete nodePtr;
}
}

Program 4
42
// This program demonstrates the deleteNode member function
#include "FloatList.h“
void main(void)
{
FloatList list;
// Build the list
cout << "Here are the initial values:n";
list.displayList();
cout << endl;
cout << "Now deleting the node in the middle.n";
cout << "Here are the nodes left.n";
list.deleteNode(7.9);
list.displayList();
cout << endl; Continued on next slide…

43
cout << "Now deleting the last node.n";
list.displayList();
cout << endl;
cout << "Now deleting the only remaining node.n";
list.displayList();
}

44
Program Output
Here are the initial values:
2.5
7.9
12.6
Now deleting the node in the middle.
Here are the nodes left.
2.5
12.6
Now deleting the last node.
2.5
Now deleting the only remaining node.

45
Look at the else part of the second if statement. This is where the function will perform
its action since the list is not empty, and the first node does not contain the value 7.9. Just
like insertNode, this function uses nodePtr and previousNode to traverse the list.
The while loop terminates when the value 7.9 is located. At this point, the list and the other
pointers will be in the state depicted in the figure below.

46
next, the following statement executes.
The statement above causes the links in the list to bypass the node that nodePtr points to.
Although the node still exists in memory, this removes it from the list.
The last statement uses the delete operator to complete the total deletion of the node.

Destroying the List
• The class's destructor should release all the
memory used by the list.
• It does so by stepping through the list,
deleting each node one-by-one. The code is
shown on the next slide.
47

48
FloatList::~FloatList(void)
{
ListNode *nodePtr, *nextNode;
nodePtr = head;
while (nodePtr != NULL)
{
nextNode = nodePtr->next;
delete nodePtr;
nodePtr = nextNode;
}
}
Notice the use of nextNode instead of previousNode. The nextNode pointer is used
to hold the position of the next node in the list, so it will be available after the node
pointed to by nodePtr is deleted.

A Linked List Template
49
#ifndef LINKEDLIST_H
#define LINKEDLIST_H
template <class T>
class LinkedList
{
private:
// Declare a structure for the list
struct ListNode
{
T value;
};
ListNode *head; // List head pointer

50
public:
LinkedList(void) // Constructor
{ head = NULL; }
~LinkedList(void); // Destructor
void appendNode(T);
void insertNode(T);
void deleteNode(T);
void displayList(void);
};
// appendNode appends a node containing the
// value pased into num, to the end of the list.
template <class T>
void LinkedList<T>::AppendNode(T num)
{
ListNode *newNode, *nodePtr;
// Allocate a new node & store num

51
if (!head)
head = newNode;
{
nodePtr = head;
}
}

52
// DisplayList shows the value
// stored in each node of the linked list
// pointed to by head.
template <class T>
void LinkedList<T>::DisplayList(void)
{
ListNode *nodePtr;
nodePtr = head;
while (nodePtr)
{
cout << nodePtr->value << endl;
}
}

53
// The insertNode function inserts a node with
// num copied to its value member.
template <class T>
void LinkedList<T>::insertNode(T num)
{
ListNode *newNode, *nodePtr, *previousNode;
// Allocate a new node & store Num
if (!head)
{
head = newNode;

54
{
nodePtr = head;
// than num.
{
}
// Insert the node after the one pointed to
// by previousNode and before the one pointed to
// by nodePtr.
previousNode->next = newNode;
newNode->next = nodePtr;
}

55
// The deleteNode function searches for a node
// with Num as its value. The node, if found, is
// deleted from the list and from memory.
template <class T>
void LinkedList<T>::deleteNode(T num)
{
ListNode *nodePtr, *previousNode;
// If the list is empty, do nothing.
if (!head)
return;
// Determine if the first node is the one.
if (head->value == num)
{
nodePtr = head->next;
delete head;
head = nodePtr;

56
else
{
nodePtr = head;
// Skip all nodes whose value member is
// not equal to num.
while (nodePtr != NULL && nodePtr->value != num)
{
}
// Link the previous node to the node after
// nodePtr, then delete nodePtr.
delete nodePtr;
}
}

57
// Destructor
// This function deletes every node in the list.
template <class T>
LinkedList<T>::~LinkedList(void)
{
ListNode *nodePtr, *nextNode;
nodePtr = head;
while (nodePtr != NULL)
{
nextNode = nodePtr->next;
delete nodePtr;
nodePtr = nextNode;
}
}
#endif

Program 5
58
// This program demonstrates the linked list template.
#include "LinkedList.h“
void main(void)
{
LinkedList<int> list;
// Build the list
list.appendNode(2);
list.appendNode(4);
list.appendNode(6);
cout << "Here are the initial values:n";
list.displayList();
cout << endl;

59
cout << "Now inserting the value 5.n";
list.insertNode(5);
cout << "Here are the nodes now.n";
list.displayList();
cout << endl;
cout << "Now deleting the last node.n";
list.deleteNode(6);
list.displayList();
}

60
Program Output
Here are the initial values:
2
4
6
Now inserting the value 5.
Here are the nodes now.
2
4
5
6
Now deleting the last node.
2
4
5

Variations of the Linked List
61
The Doubly-Linked List

62
Variations of the Linked List
The Circular Linked List

The STL list Container
• The list container, found in the Standard Template
Library, is a template version of a doubly linked list.
• STL lists can insert elements, or add elements to
their front quicker than vectors can, because
lists do not have to shift the other elements.
• lists are also efficient at adding elements at their
back because they have a built-in pointer to the last
element in the list (no traversal required).
63

64
Member Function Examples & Description
back cout << list.back() << endl;
The back member function returns a reference to the
last element in the list.
erase list.erase(iter);
list.erase(firstIter, lastIter)
The first example causes the list element pointed to by the iterator
iter to be removed. The second example causes all of the list
elements from firstIter to lastIter to be removed.
empty if (list.empty())
The empty member function returns true if the list is empty. If
the list has elements, it returns false.

65
end iter = list.end();
end returns a bi-directional iterator to the end of the list.
front cout << list.front() << endl;
front returns a reference to the first element of the list.
insert list.insert(iter, x)
The insert member function inserts an element into the list.
The
example shown above inserts an element with the value x, just
before
the element pointed to by iter.
merge list1.merge(list2);
merge inserts all the items in list2 into list1. list1 is
expanded to accommodate the new elements plus any elements
already stored in list1. merge expects both lists to be sorted.
When list2 is inserted into list1, the elements are inserted into
their correct position, so the resulting list is also sorted.

66
pop_back list.pop_back();
pop_back removes the last element of the list.
pop_front list.pop_front();
pop_front removes the first element of the list.
push_back list.push_back(x);
push_back inserts an element with value x at the end of
the list.
push_front list.push_front(x);
push_front inserts an element with value x at the beginning of
the
list.
reverse list.reverse();
reverse reverses the order in which the elements appear in the
list.

67
size() Returns the number of elements in the list.
swap list1.swap(List2)
The swap member function swaps the elements stored in two
lists. For example, assuming list1 and list2 are lists, the
statement shown above will exchange the values in the two.
unique list.unique();
unique removes any element that has the same value as the
element
before it.

Program 6
68
// This program demonstrates the STL list container.
#include <list> // Include the list header
using namespace std; // Required by some compilers
void main(void)
{
list<int> myList;
list<int>::iterator iter;
// Add values to the list
for (int x = 0; x < 100; x += 10)
myList.push_back(x);
// Display the values
for (iter = myList.begin(); iter != myList.end(); iter++)
cout << *iter << " ";
cout << endl;

69
// Now reverse the order of the elements
myList.reverse();
// Display the values again
for (iter = myList.begin(); iter != myList.end(); iter++)
cout << *iter << " ";
cout << endl;
}
Program Output
0 10 20 30 40 50 60 70 80 90
90 80 70 60 50 40 30 20 10 0

Linked lists in Data Structure

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