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 classclass 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 classclass 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 beginningdef 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 abovedef 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 abovedef 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 classclass 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 objectclass 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 hereif __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!
