# Python Program For Inserting A Node In A Linked List

• Difficulty Level : Basic
• Last Updated : 05 Sep, 2022

We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.
All programs discussed in this post consider the following representations of linked list.

## Python

 `# Node class``class` `Node:` `    ``# Function to initialize the``    ``# node object``    ``def` `__init__(``self``, data):` `        ``# Assign data``        ``self``.data ``=` `data ``  ` `        ``# Initialize next as null``        ``self``.``next` `=` `None`  `# Linked List class``class` `LinkedList:``  ` `    ``# Function to initialize the``    ``# Linked List object``    ``def` `__init__(``self``):``        ``self``.head ``=` `None`

In this post, methods to insert a new node in linked list are discussed. A node can be added in three ways
1) At the front of the linked list
2) After a given node.
3) At the end of the linked list.

Add a node at the front: (4 steps process)

Following are the 4 steps to add a node at the front.

## Python

 `# This function is in LinkedList class``# Function to insert a new node at``# the beginning``def` `push(``self``, new_data):` `    ``# 1 & 2: Allocate the Node &``    ``#        Put in the data``    ``new_node ``=` `Node(new_data)``        ` `    ``# 3. Make next of new Node as head``    ``new_node.``next` `=` `self``.head``        ` `    ``# 4. Move the head to point to new Node``    ``self``.head ``=` `new_node`

Time complexity of push() is O(1) as it does a constant amount of work.
Add a node after a given node: (5 steps process)
We are given a pointer to a node, and the new node is inserted after the given node.

## Python

 `# This function is in LinkedList class.``# Inserts a new node after the given``# prev_node. This method is defined``# inside LinkedList class shown above``def` `insertAfter(``self``, prev_node, new_data):` `    ``# 1. Check if the given prev_node exists``    ``if` `prev_node ``is` `None``:``        ``print` `"The given previous node must in LinkedList."``        ``return` `    ``# 2. Create new node &``    ``# 3. Put in the data``    ``new_node ``=` `Node(new_data)` `    ``# 4. Make next of new Node as next of prev_node``    ``new_node.``next` `=` `prev_node.``next` `    ``# 5. make next of prev_node as new_node``    ``prev_node.``next` `=` `new_node`

Time complexity of insertAfter() is O(1) as it does a constant amount of work.

Add a node at the end: (6 steps process)
The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30.
Since a Linked List is typically represented by the head of it, we have to traverse the list till the end and then change the next to last node to a new node.

Following are the 6 steps to add node at the end.

## Python

 `# This function is defined in Linked List``# class appends a new node at the end. ``# This method is defined inside LinkedList``# class shown above``def` `append(``self``, new_data):` `   ``# 1. Create a new node``   ``# 2. Put in the data``   ``# 3. Set next as None``   ``new_node ``=` `Node(new_data)` `   ``# 4. If the Linked List is empty, then``   ``#    make the new node as head``   ``if` `self``.head ``is` `None``:``        ``self``.head ``=` `new_node``        ``return` `   ``# 5. Else traverse till the last node``   ``last ``=` `self``.head``   ``while` `(last.``next``):``       ``last ``=` `last.``next` `   ``# 6. Change the next of last node``   ``last.``next` `=`  `new_node`

Time complexity of append is O(n) where n is the number of nodes in the linked list. Since there is a loop from head to end, the function does O(n) work.
This method can also be optimized to work in O(1) by keeping an extra pointer to the tail of the linked list/

Following is a complete program that uses all of the above methods to create a linked list.

## Python

 `# A complete working Python program to demonstrate all``# insertion methods of linked list` `# Node class``class` `Node:` `    ``# Function to initialize the``    ``# node object``    ``def` `__init__(``self``, data):` `        ``# Assign data``        ``self``.data ``=` `data ` `        ``# Initialize next as null``        ``self``.``next` `=` `None`   `# Linked List class contains a``# Node object``class` `LinkedList:` `    ``# Function to initialize head``    ``def` `__init__(``self``):``        ``self``.head ``=` `None` `    ``# Functio to insert a new node at``    ``# the beginning``    ``def` `push(``self``, new_data):` `        ``# 1 & 2: Allocate the Node &``        ``#        Put in the data``        ``new_node ``=` `Node(new_data)` `        ``# 3. Make next of new Node as head``        ``new_node.``next` `=` `self``.head` `        ``# 4. Move the head to point to new Node``        ``self``.head ``=` `new_node`  `    ``# This function is in LinkedList class.``    ``# Inserts a new node after the given``    ``# prev_node. This method is defined``    ``# inside LinkedList class shown above``    ``def` `insertAfter(``self``, prev_node, new_data):` `        ``# 1. Check if the given prev_node exists``        ``if` `prev_node ``is` `None``:``            ``print` `"The given previous node must inLinkedList."``            ``return` `        ``#  2. Create new node &``        ``#     Put in the data``        ``new_node ``=` `Node(new_data)` `        ``# 4. Make next of new Node as next``        ``#    of prev_node``        ``new_node.``next` `=` `prev_node.``next` `        ``# 5. make next of prev_node as new_node``        ``prev_node.``next` `=` `new_node`  `    ``# This function is defined in Linked List class``    ``# Appends a new node at the end.  This method is``    ``# defined inside LinkedList class shown above */``    ``def` `append(``self``, new_data):` `        ``# 1. Create a new node``        ``# 2. Put in the data``        ``# 3. Set next as None``        ``new_node ``=` `Node(new_data)` `        ``# 4. If the Linked List is empty, then make the``        ``#    new node as head``        ``if` `self``.head ``is` `None``:``            ``self``.head ``=` `new_node``            ``return` `        ``# 5. Else traverse till the last node``        ``last ``=` `self``.head``        ``while` `(last.``next``):``            ``last ``=` `last.``next` `        ``# 6. Change the next of last node``        ``last.``next` `=`  `new_node` `    ``# Utility function to print the``    ``# linked list``    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while` `(temp):``            ``print` `temp.data,``            ``temp ``=` `temp.``next` `# Code execution starts here``if` `__name__``=``=``'__main__'``:` `    ``# Start with the empty list``    ``llist ``=` `LinkedList()` `    ``# Insert 6.  So linked list``    ``becomes ``6``-``>``None``    ``llist.append(``6``)` `    ``# Insert 7 at the beginning. So``    ``# linked list becomes 7->6->None``    ``llist.push(``7``);` `    ``# Insert 1 at the beginning. So``    ``# linked list becomes 1->7->6->None``    ``llist.push(``1``);` `    ``# Insert 4 at the end. So linked list``    ``# becomes 1->7->6->4->None``    ``llist.append(``4``)` `    ``# Insert 8, after 7. So linked list``    ``# becomes 1 -> 7-> 8-> 6-> 4-> None``    ``llist.insertAfter(llist.head.``next``, ``8``)` `    ``print` `'Created linked list is:'``,``    ``llist.printList()``# This code is contributed by Manikantan Narasimhan`

Output:

` Created Linked list is:  1  7  8  6  4`

Time complexity: O(N) where N is size of given linked list

Auxiliary space: O(1), it is not taking extra space

Please refer complete article on Linked List | Set 2 (Inserting a node) for more details!

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