Python Program For Inserting A Node In A Linked List

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)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example, if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)

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