Linked Lists Geletaw S.. Objective At the end of the session students should have basic...

Linked Lists

Geletaw S.

Objective

At the end of the session students should have

basic understanding of

– Linked lists

– Basic operations of linked lists• Insert, find, delete, print, etc.

– Variations of linked lists• Simple Linked Lists

• Circular Linked Lists

• Doubly Linked Lists

Array vs Linked List

node

node nodeArray

Linked List

What’s wrong with Array ?

Disadvantages of arrays as storage data structures

– slow searching in unordered array

– slow insertion in ordered array

– Fixed size

Why lists (Linked lists )?• Solve some of these problems

• General purpose storage data structures and are versatile.

• More complex to code and manage than arrays, but they have some distinct

advantages.

• Dynamic: a linked list can easily grow and shrink in size.

– We don’t need to know how many nodes will be in the list. They are

created in memory as needed.

– In contrast, the size of a C++ array is fixed at compilation time.

• Easy and fast insertions and deletions

– To insert or delete an element in an array, we need to copy to temporary variables

to make room for new elements or close the gap caused by deleted elements.

– With a linked list, no need to move other nodes. Only need to reset some pointers.

Linked Lists

• Each data item is embedded in a link.

• Each Link object contains a reference to the next link in

the list of items.

In an array

– items have a particular position, identified by its index.

In a list

– the only way to access an item is to traverse the list

Linked Lists

• A linked list is a series of connected nodes• Each node contains at least

– A piece of data (any type)– Pointer to the next node in the list

• Head: pointer to the first node• The last node points to NULL

A

Head

B C

A

data pointer

node

A Simple Linked List Class Operations of List

IsEmpty: – determine whether or not the list is empty

InsertNode: – insert a new node at a particular position

FindNode: – find a node with a given value

DeleteNode: – delete a node with a given value

DisplayList: – print all the nodes in the list

A Simple Linked List Class

• We use two classes: Node and List• Declare Node class for the nodes

– data: double-type data in this example– next: a pointer to the next node in the list

class Node {public:

double data; // dataNode* next; // pointer to next

};

A Simple Linked List Class• Declare List, which contains

– head: a pointer to the first node in the list. Since the list is empty initially, head is set to NULL– Operations on List

class List {public:

List(void) { head = NULL; } // constructor~List(void); // destructor

bool IsEmpty() { return head == NULL; }Node* InsertNode(int index, double x);int FindNode(double x);int DeleteNode(double x);void DisplayList(void);

private:Node* head;

};

Inserting a new node

• Node* InsertNode(int index, double x) – Insert a node with data equal to x after the index’th elements.

(i.e., when index = 0, insert the node as the first element; when index = 1, insert the node after the first element, and so on)

– If the insertion is successful,return the inserted node.

– Otherwise, return NULL. (If index is < 0 or > length of the list, the insertion will fail.)

Steps1. Locate index’th element2. Allocate memory for the new node3. Point the new node to its successor4. Point the new node’s predecessor to the new node newNode

index’th element

Inserting a new node

Possible cases of InsertNode1. Insert into an empty list2. Insert in front3. Insert at back4. Insert in middle

But, in fact, only need to handle two cases– Insert as the first node (Case 1 and Case 2)– Insert in the middle or at the end of the list (Case 3 and

Case 4)

Inserting a new nodeNode* List::InsertNode(int index, double x) {

if (index < 0) return NULL;

int currIndex = 1;Node* currNode = head;while (currNode && index > currIndex) {

currNode = currNode->next;currIndex++;

}if (index > 0 && currNode == NULL) return NULL;

Node* newNode = new Node;newNode->data = x;if (index == 0) {

newNode->next = head;head = newNode;

}else {

newNode->next = currNode->next;currNode->next = newNode;

}return newNode;

}

Try to locate index’th node. If it doesn’t exist, return NULL.

Inserting a new nodeNode* List::InsertNode(int index, double x) {

if (index < 0) return NULL;

int currIndex = 1;Node* currNode = head;while (currNode && index > currIndex) {

currNode = currNode->next;currIndex++;

}if (index > 0 && currNode == NULL) return NULL;

Node* newNode = new Node;newNode->data = x;if (index == 0) {

newNode->next = head;head = newNode;

}else {

newNode->next = currNode->next;currNode->next = newNode;

}return newNode;

}

Create a new node

Inserting a new nodeNode* List::InsertNode(int index, double x) {

if (index < 0) return NULL;

int currIndex = 1;Node* currNode = head;while (currNode && index > currIndex) {

currNode = currNode->next;currIndex++;

}if (index > 0 && currNode == NULL) return NULL;

Node* newNode = new Node;newNode->data = x;if (index == 0) {

newNode->next = head;head = newNode;

}else {

newNode->next = currNode->next;currNode->next = newNode;

}return newNode;

}

Insert as first element

head

newNode

Inserting a new nodeNode* List::InsertNode(int index, double x) {

if (index < 0) return NULL;

int currIndex = 1;Node* currNode = head;while (currNode && index > currIndex) {

currNode = currNode->next;currIndex++;

}if (index > 0 && currNode == NULL) return NULL;

Node* newNode = new Node;newNode->data = x;if (index == 0) {

newNode->next = head;head = newNode;

}else {

newNode->next = currNode->next;currNode->next = newNode;

}return newNode;

}

Insert after currNode

newNode

currNode

Finding a node• int FindNode(double x)

– Search for a node with the value equal to x in the list.– If such a node is found, return its position. Otherwise, return 0.

int List::FindNode(double x) {Node* currNode = head;int currIndex = 1;while (currNode && currNode->data != x) {

currNode = currNode->next;currIndex++;

}if (currNode) return currIndex;return 0;

}

Deleting a node• int DeleteNode(double x)

– Delete a node with the value equal to x from the list.– If such a node is found, return its position. Otherwise, return 0.

• Steps– Find the desirable node (similar to FindNode)– Release the memory occupied by the found node– Set the pointer of the predecessor of the found node to the

successor of the found node

• Like InsertNode, there are two special cases– Delete first node– Delete the node in middle or at the end of the list

The Scenario

• Begin with an existing linked list– Could be empty or not– Could be ordered or not

4 17

head

426

The Scenario

• Begin with an existing linked list– Could be empty or not– Could be ordered or not

4 17

head

426

The Scenario

• Begin with an existing linked list– Could be empty or not– Could be ordered or not

4 17

head

42

The Scenario

• Begin with an existing linked list– Could be empty or not– Could be ordered or not

4 17

head

42

Deleting a nodeint List::DeleteNode(double x) {

Node* prevNode = NULL;Node* currNode = head;int currIndex = 1;while (currNode && currNode->data != x) {

prevNode = currNode;currNode = currNode->next;currIndex++;

}if (currNode) {

if (prevNode) {prevNode->next = currNode->next;delete currNode;

}else {

head = currNode->next;delete currNode;

}return currIndex;

}return 0;

}

Try to find the node with its value equal to x

Deleting a nodeint List::DeleteNode(double x) {

Node* prevNode = NULL;Node* currNode = head;int currIndex = 1;while (currNode && currNode->data != x) {

prevNode = currNode;currNode = currNode->next;currIndex++;

}if (currNode) {

if (prevNode) {prevNode->next = currNode->next;delete currNode;

}else {

head = currNode->next;delete currNode;

}return currIndex;

}return 0;

}

currNodeprevNode

Deleting a nodeint List::DeleteNode(double x) {

Node* prevNode = NULL;Node* currNode = head;int currIndex = 1;while (currNode && currNode->data != x) {

prevNode = currNode;currNode = currNode->next;currIndex++;

}if (currNode) {

if (prevNode) {prevNode->next = currNode->next;delete currNode;

}else {

head = currNode->next;delete currNode;

}return currIndex;

}return 0;

}

currNodehead

Printing all the elements

• void DisplayList(void)– Print the data of all the elements – Print the number of the nodes in the list

void List::DisplayList(){ int num = 0; Node* currNode = head; while (currNode != NULL){

cout << currNode->data << endl;currNode = currNode->next;num++;

} cout << "Number of nodes in the list: " << num << endl;}

Destroying the list

• ~List(void)– Use the destructor to release all the memory used by the list.– Step through the list and delete each node one by one.

List::~List(void) { Node* currNode = head, *nextNode = NULL; while (currNode != NULL) {

nextNode = currNode->next;// destroy the current nodedelete currNode;currNode = nextNode;

}}

Using Listint main(void){

List list;list.InsertNode(0, 7.0); // successfullist.InsertNode(1, 5.0); // successfullist.InsertNode(-1, 5.0); // unsuccessfullist.InsertNode(0, 6.0); // successfullist.InsertNode(8, 4.0); // unsuccessful// print all the elementslist.DisplayList();if(list.FindNode(5.0) > 0) cout << "5.0 found" << endl;else cout << "5.0 not found" << endl;if(list.FindNode(4.5) > 0) cout << "4.5 found" << endl;else cout << "4.5 not found" << endl;list.DeleteNode(7.0);list.DisplayList();return 0;

}

675Number of nodes in the list: 35.0 found4.5 not found65Number of nodes in the list: 2

result

Linked List Efficiency• Insertion and deletion at the beginning of the list are very

fast, O(1).• Finding, deleting or inserting in the list requires searching

through half the items in the list on an average, requiring O(n) comparisons.

• Although arrays require same number of comparisons, the advantage lies in the fact that no items need to be moved after insertion or deletion.

• As opposed to fixed size of arrays, linked lists use exactly as much memory as is needed and can expand.

More on Linked Lists

list

is a sequential container optimized for insertion and

erasure at arbitrary points in the sequence

Linked List

Each data item is embedded in a link & Each Link object

contains a reference to the next link in the list of items.

Summary

Each node in a circular linked list has a predecessor (and a successor), provided that the list is nonempty.

insertion and deletion do not require special consideration of the first node.

This is a good implementation for a linked queue or for any problem in which "circular" processing is required

last

9 17 22 26 34

In other applications a circular linked list is used; instead of the last node containing a null pointer, it contains a pointer to the first node in the list.

For such lists,one can use a single pointer to the last node in the list, because then one has direct access to it and "almost-direct" access to the first node.

Circular Linked List

Node* prevNode = NULL;Node* currNode = head;int currIndex = 1;while (currNode && currNode->data != x)

{prevNode = currNode;currNode = currNode->next;currIndex++;

}

Inserting a new nodeNode* List::InsertNode(int index, double x) {

if (index < 0) return NULL;

int currIndex = 1;Node* currNode = head;while (currNode && index > currIndex) {

prevnode=currnodecurrNode = currNode->next;currIndex++;

}if (index > 0 && currNode == NULL) return NULL;

Node* newNode = new Node;newNode->data = x;if (index == 0) {

newNode->next = head;head = newNode;

}else {

newNode->next = currNode->next;currNode->next = newNode;

}return newNode;

}

Try to locate index’th node. If it doesn’t exist, return NULL.

Circularly Linked ListsFor example, item can be inserted as follows:

newptr = new Node(item, 0);

if (Index == 0) // list is empty

{

newptr->next = newptr;

currentIndex = newptr;

}

else // nonempty list

{

newptr->next = predptr->next;

predptr->next = newptr;

}

Note that a one-element circularly linked list points to itself.

Circularly Linked ListsTraversal must be modified: avoid an infinite loop by looking for the

end of list as signalled by a null pointer.

Like other methods, deletion must also be slightly modified.

Deleting the last node is signalled when the node deleted points to itself.

if (Index == 0) // list is empty

// Signal that the list is empty

else

{

ptr = predptr->next; // hold node for deletion

if (ptr == predptr) // one-node list

Index = 0;

else // list with 2 or more nodes

predptr->next = ptr->next;

delete ptr;

}

Summary

In Circular linked lists– The last node points to the first node of the list

– How do we know when we have finished traversing the list? (Tip: check if the pointer of the current node is equal to the head.)

A

Head

B C

Double Linked Lists

All of these lists, however, are uni-directional; we can only move from one node to its successor.

last

prev L

first

mySize 5

9 17 22 26 34

next

In many applications, bidirectional movement is necessary. In this case, each node has two pointers — one to its successor (null if there is none) and one to its predecessor (null if there is none.) Such a list is commonly called a doubly-linked (or symmetrically-linked) list.

Doubly Linked Lists

Con’t….• Similar to an ordinary list with the addition that a link

to the last item is maintained along with that to the first.

• The reference to the last link permits to insert a new link directly at the end of the list as well as at the beginning.

• This could not be done in the ordinary linked list without traversing the whole list.

• This technique is useful in implementing the Queue where insertions are made at end and deletions from the front.

Linked Lists 41

Insertion• We visualize operation insertAfter(p, X), which returns position q

A B X C

A B C

p

A B C

p

X

q

p q

Linked Lists 42

Insertion AlgorithmAlgorithm insertAfter(p,e):

Create a new node v

v.setElement(e)

v.setPrev(p) {link v to its predecessor}

v.setNext(p.getNext()) {link v to its successor}

(p.getNext()).setPrev(v) {link p’s old successor to v}

p.setNext(v) {link p to its new successor, v}

return v {the position for the element e}

Linked Lists 43

Deletion• We visualize remove(p), where p == last()

A B C D

p

A B C

D

p

A B C

Linked Lists 44

Deletion AlgorithmAlgorithm remove(p):

t = p.element {a temporary variable to hold the

return value}

(p.getPrev()).setNext(p.getNext()) {linking out p}

(p.getNext()).setPrev(p.getPrev())

p.setPrev(null){invalidating the position p}

p.setNext(null)

return t

Linked Lists 45

Worst-cast running time

• In a doubly linked list+ insertion at head or tail is in O(1)+ deletion at either end is on O(1)-- element access is still in O(n)

Summary

– Each node points to not only successor but the predecessor

– There are two NULL: at the first and last nodes in the list

– Advantage: given a node, it is easy to visit its predecessor. Convenient to traverse lists backwards

Linked Lists 47

Summary

• A doubly linked list is often more convenient!

• Nodes store:– element– link to the previous node– link to the next node

• Special trailer and header nodes

prev next

elem

trailerheader nodes/positions

elements

node

Con’t….

L

first

mySize 5

last9 17 22 26 34

prev

next

doubly-linked list => bidirectional movement!!

Each node has two pointers — one to its successor (null if there is none) and one to its predecessor (null if there is none.)

Doubly linked lists give one more flexibility (can move in either direction)

BUT at significant cost :

DOUBLE the overhead for links

More complex code

The End !!!