Python Linked List

In this article, we will learn about the implementation of a linked list in Python. To implement the linked list in Python, we will use classes in Python. Now, we know that a linked list consists of nodes and nodes have two elements i.e. data and a reference to another node. Let’s implement the node first. 

What is Linked List in Python

A linked list is a type of linear data structure similar to arrays. It is a collection of nodes that are linked with each other. A node contains two things first is data and second is a link that connects it with another node. Below is an example of a linked list with four nodes and each node contains character data and a link to another node. Our first node is where head points and we can access all the elements of the linked list using the head.

Linked List

Creating a linked list in Python

In this LinkedList class, we will use the Node class to create a linked list. In this class, we have an __init__ method that initializes the linked list with an empty head. Next, we have created an insertAtBegin() method to insert a node at the beginning of the linked list, an insertAtIndex() method to insert a node at the given index of the linked list, and insertAtEnd() method inserts a node at the end of the linked list. After that, we have the remove_node() method which takes the data as an argument to delete that node. In the remove_node() method we traverse the linked list if a node is present equal to data then we delete that node from the linked list. Then we have the sizeOfLL() method to get the current size of the linked list and the last method of the LinkedList class is printLL() which traverses the linked list and prints the data of each node.

Creating a Node Class

We have created a Node class in which we have defined a __init__ function to initialize the node with the data passed as an argument and a reference with None because if we have only one node then there is nothing in its reference.

Python3

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

Insertion in Linked List

Insertion at Beginning in Linked List 

This method inserts the node at the beginning of the linked list. In this method, we create a new_node with the given data and check if the head is an empty node or not if the head is empty then we make the new_node as head and return else we insert the head at the next new_node and make the head equal to new_node.

Python3

def insertAtBegin(self, data):
    new_node = Node(data)
    if self.head is None:
        self.head = new_node
        return
    else:
        new_node.next = self.head
        self.head = new_node

Insert a Node at a Specific Position in a Linked List

This method inserts the node at the given index in the linked list. In this method, we create a new_node with given data , a current_node that equals to the head, and a counter ‘position’ initializes with 0. Now, if the index is equal to zero it means the node is to be inserted at begin so we called insertAtBegin() method else we run a while loop until the current_node is not equal to None or (position+1) is not equal to the index we have to at the one position back to insert at a given position to make the linking of nodes and in each iteration, we increment the position by 1 and make the current_node next of it. When the loop breaks and if current_node is not equal to None we insert new_node at after to the current_node. If current_node is equal to None it means that the index is not present in the list and we print “Index not present”.

Python3

# Indexing starts from 0.
def insertAtIndex(self, data, index):
        new_node = Node(data)
        current_node = self.head
        position = 0
        if position == index:
            self.insertAtBegin(data)
        else:
            while(current_node != None and position+1 != index):
                position = position+1
                current_node = current_node.next

            if current_node != None:

                new_node.next = current_node.next
                current_node.next = new_node
            else:
                print("Index not present")

Insertion in Linked List at End

This method inserts the node at the end of the linked list. In this method, we create a new_node with the given data and check if the head is an empty node or not if the head is empty then we make the new_node as head and return else we make a current_node equal to the head traverse to the last node of the linked list and when we get None after the current_node the while loop breaks and insert the new_node in the next of current_node which is the last node of linked list.

Python3

def inserAtEnd(self, data):
    new_node = Node(data)
    if self.head is None:
        self.head = new_node
        return

    current_node = self.head
    while(current_node.next):
        current_node = current_node.next

    current_node.next = new_node

Update the Node of a Linked List

This code defines a method called updateNode in a linked list class. It is used to update the value of a node at a given position in the linked list.

Python3

# Update node of a linked list
# at given position
def updateNode(self, val, index):
    current_node = self.head
    position = 0
    if position == index:
        current_node.data = val
    else:
        while(current_node != None and position != index):
            position = position+1
            current_node = current_node.next

        if current_node != None:
            current_node.data = val
        else:
            print("Index not present")

Delete Node in a Linked List

Remove First Node from Linked List

This method removes the first node of the linked list simply by making the second node head of the linked list.

Python3

def remove_first_node(self):
    if(self.head == None):
        return

    self.head = self.head.next

Remove Last Node from Linked List

In this method, we will delete the last node. First, we traverse to the second last node using the while loop, and then we make the next of that node None and last node will be removed.

Python3

def remove_last_node(self):

    if self.head is None:
        return

    current_node = self.head
    while(current_node.next.next):
        current_node = current_node.next

    current_node.next = None

Delete a Linked List Node at a given Position

In this method, we will remove the node at the given index, this method is similar to the insert_at_inded() method. In this method, if the head is None we simply return else we initialize a current_node with self.head and position with 0. If the position is equal to the index we called the remove_first_node() method else we traverse to the one node before that we want to remove using the while loop. After that when we out of the while loop we check that current_node is equal to None if not then we make the next of current_node equal to the next of node that we want to remove else we print the message “Index not present” because current_node is equal to None.

Python3

# Method to remove at given index
def remove_at_index(self, index):
        if self.head == None:
            return

        current_node = self.head
        position = 0
        if position == index:
            self.remove_first_node()
        else:
            while(current_node != None and position+1 != index):
                position = position+1
                current_node = current_node.next

            if current_node != None:
                current_node.next = current_node.next.next
            else:
                print("Index not present")

Delete a Linked List Node of a given Data

This method removes the node with the given data from the linked list. In this method, firstly we made a current_node equal to the head and run a while loop to traverse the linked list. This while loop breaks when current_node becomes None or the data next to the current node is equal to the data given in the argument. Now, After coming out of the loop if the current_node is equal to None it means that the node is not present in the data and we just return, and if the data next to the current_node is equal to the data given then we remove that node by making next of that removed_node to the next of current_node. And this is implemented using the if else condition.

Python3

def remove_node(self, data):
    current_node = self.head

    # Check if the head node contains the specified data
    if current_node.data == data:
        self.remove_first_node()
        return

    while current_node is not None and current_node.next.data != data:
        current_node = current_node.next

    if current_node is None:
        return
    else:
        current_node.next = current_node.next.next

Linked List Traversal in Python

This method traverses the linked list and prints the data of each node. In this method, we made a current_node equal to the head and iterate through the linked list using a while loop until the current_node become None and print the data of current_node in each iteration and make the current_node next to it.

Python3

def printLL(self):
    current_node = self.head
    while(current_node):
        print(current_node.data)
        current_node = current_node.next

Get Length of a Linked List in Python

This method returns the size of the linked list. In this method, we have initialized a counter ‘size’ with 0, and then if the head is not equal to None we traverse the linked list using a while loop and increment the size with 1 in each iteration and return the size when current_node becomes None else we return 0.

Python3

def sizeOfLL(self):
    size = 0
    if(self.head):
        current_node = self.head
        while(current_node):
            size = size+1
            current_node = current_node.next
        return size
    else:
        return 0

Example of the Linked list in Python

In this example, After defining the Node and LinkedList class we have created a linked list named “llist” using the linked list class and then insert four nodes with character data ‘a’, ‘b’, ‘c’, ‘d’ and ‘g’ in the linked list then we print the linked list using printLL() method linked list class after that we have removed some nodes using remove methods and then print the linked list again and we can see in the output that node is deleted successfully. After that, we also print the size of the linked list.

Python3

# Create a Node class to create a node
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

# Create a LinkedList class


class LinkedList:
    def __init__(self):
        self.head = None

    # Method to add a node at begin of LL
    def insertAtBegin(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
            return
        else:
            new_node.next = self.head
            self.head = new_node

    # Method to add a node at any index
    # Indexing starts from 0.
    def insertAtIndex(self, data, index):
        new_node = Node(data)
        current_node = self.head
        position = 0
        if position == index:
            self.insertAtBegin(data)
        else:
            while(current_node != None and position+1 != index):
                position = position+1
                current_node = current_node.next

            if current_node != None:
                new_node.next = current_node.next
                current_node.next = new_node
            else:
                print("Index not present")

    # Method to add a node at the end of LL

    def insertAtEnd(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
            return

        current_node = self.head
        while(current_node.next):
            current_node = current_node.next

        current_node.next = new_node

    # Update node of a linked list
        # at given position
    def updateNode(self, val, index):
        current_node = self.head
        position = 0
        if position == index:
            current_node.data = val
        else:
            while(current_node != None and position != index):
                position = position+1
                current_node = current_node.next

            if current_node != None:
                current_node.data = val
            else:
                print("Index not present")

    # Method to remove first node of linked list

    def remove_first_node(self):
        if(self.head == None):
            return

        self.head = self.head.next

    # Method to remove last node of linked list
    def remove_last_node(self):

        if self.head is None:
            return

        current_node = self.head
        while(current_node.next.next):
            current_node = current_node.next

        current_node.next = None

    # Method to remove at given index
    def remove_at_index(self, index):
        if self.head == None:
            return

        current_node = self.head
        position = 0
        if position == index:
            self.remove_first_node()
        else:
            while(current_node != None and position+1 != index):
                position = position+1
                current_node = current_node.next

            if current_node != None:
                current_node.next = current_node.next.next
            else:
                print("Index not present")

    # Method to remove a node from linked list
    def remove_node(self, data):
        current_node = self.head

        if current_node.data == data:
            self.remove_first_node()
            return

        while(current_node != None and current_node.next.data != data):
            current_node = current_node.next

        if current_node == None:
            return
        else:
            current_node.next = current_node.next.next

    # Print the size of linked list
    def sizeOfLL(self):
        size = 0
        if(self.head):
            current_node = self.head
            while(current_node):
                size = size+1
                current_node = current_node.next
            return size
        else:
            return 0

    # print method for the linked list
    def printLL(self):
        current_node = self.head
        while(current_node):
            print(current_node.data)
            current_node = current_node.next


# create a new linked list
llist = LinkedList()

# add nodes to the linked list
llist.insertAtEnd('a')
llist.insertAtEnd('b')
llist.insertAtBegin('c')
llist.insertAtEnd('d')
llist.insertAtIndex('g', 2)

# print the linked list
print("Node Data")
llist.printLL()

# remove a nodes from the linked list
print("\nRemove First Node")
llist.remove_first_node()
print("Remove Last Node")
llist.remove_last_node()
print("Remove Node at Index 1")
llist.remove_at_index(1)


# print the linked list again
print("\nLinked list after removing a node:")
llist.printLL()

print("\nUpdate node Value")
llist.updateNode('z', 0)
llist.printLL()

print("\nSize of linked list :", end=" ")
print(llist.sizeOfLL())

Output

Node Data
c
a
g
b
d

Remove First Node
Remove Last Node
Remove Node at Index 1

Linked list after removing a node:
a
b

Update node Value
z
b

Size of linked list : 2