Python Program For Removing Duplicates From A Sorted Linked List

• Difficulty Level : Expert
• Last Updated : 19 May, 2022

Write a function that takes a list sorted in non-decreasing order and deletes any duplicate nodes from the list. The list should only be traversed once.
For example if the linked list is 11->11->11->21->43->43->60 then removeDuplicates() should convert the list to 11->21->43->60.

Algorithm:
Traverse the list from the head (or start) node. While traversing, compare each node with its next node. If the data of the next node is the same as the current node then delete the next node. Before we delete a node, we need to store the next pointer of the node

Implementation:
Functions other than removeDuplicates() are just to create a linked list and test removeDuplicates().

Python3

 `# Python3 program to remove duplicate ` `# nodes from a sorted linked list  ` ` `  `# Node class  ` `class` `Node:  ` ` `  `    ``# Constructor to initialize  ` `    ``# the node object  ` `    ``def` `__init__(``self``, data):  ` `        ``self``.data ``=` `data  ` `        ``self``.``next` `=` `None` ` `  `class` `LinkedList:  ` ` `  `    ``# Function to initialize head  ` `    ``def` `__init__(``self``):  ` `        ``self``.head ``=` `None` ` `  `    ``# Function to insert a new node  ` `    ``# at the beginning  ` `    ``def` `push(``self``, new_data):  ` `        ``new_node ``=` `Node(new_data)  ` `        ``new_node.``next` `=` `self``.head  ` `        ``self``.head ``=` `new_node  ` ` `  `    ``# Given a reference to the head of a  ` `    ``# list and a key, delete the first  ` `    ``# occurrence of key in linked list  ` `    ``def` `deleteNode(``self``, key):  ` `         `  `        ``# Store head node  ` `        ``temp ``=` `self``.head  ` ` `  `        ``# If head node itself holds the  ` `        ``# key to be deleted  ` `        ``if` `(temp ``is` `not` `None``):  ` `            ``if` `(temp.data ``=``=` `key):  ` `                ``self``.head ``=` `temp.``next` `                ``temp ``=` `None` `                ``return` ` `  `        ``# Search for the key to be deleted,  ` `        ``# keep track of the previous node as ` `        ``# we need to change 'prev.next'  ` `        ``while``(temp ``is` `not` `None``):  ` `            ``if` `temp.data ``=``=` `key:  ` `                ``break` `            ``prev ``=` `temp  ` `            ``temp ``=` `temp.``next` ` `  `        ``# If key was not present in  ` `        ``# linked list  ` `        ``if``(temp ``=``=` `None``):  ` `            ``return` ` `  `        ``# Unlink the node from linked list  ` `        ``prev.``next` `=` `temp.``next` ` `  `        ``temp ``=` `None` ` `  `    ``# Utility function to print the  ` `    ``# linked LinkedList  ` `    ``def` `printList(``self``):  ` `        ``temp ``=` `self``.head  ` `        ``while``(temp):  ` `            ``print``(temp.data , end ``=` `' '``) ` `            ``temp ``=` `temp.``next` `     `  `    ``# This function removes duplicates  ` `    ``# from a sorted list          ` `    ``def` `removeDuplicates(``self``): ` `        ``temp ``=` `self``.head ` `        ``if` `temp ``is` `None``: ` `            ``return` `        ``while` `temp.``next` `is` `not` `None``: ` `            ``if` `temp.data ``=``=` `temp.``next``.data: ` `                ``new ``=` `temp.``next``.``next` `                ``temp.``next` `=` `None` `                ``temp.``next` `=` `new ` `            ``else``: ` `                ``temp ``=` `temp.``next` `        ``return` `self``.head ` ` `  `# Driver Code  ` `llist ``=` `LinkedList()  ` ` `  `llist.push(``20``)  ` `llist.push(``13``)  ` `llist.push(``13``) ` `llist.push(``11``) ` `llist.push(``11``) ` `llist.push(``11``) ` `print` `(``"Created Linked List: "``) ` `llist.printList()  ` `print``() ` `print``(``"Linked List after removing"``,  ` `      ``"duplicate elements:"``) ` `llist.removeDuplicates() ` `llist.printList()  ` `# This code is contributed by Dushyant Pathak. `

Output:

```Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20```

Time Complexity: O(n) where n is the number of nodes in the given linked list.

Recursive Approach :

Python3

 `# Python3 Program to remove duplicates ` `# from a sorted linked list  ` `import` `math ` ` `  `# Link list node  ` `class` `Node:  ` `    ``def` `__init__(``self``, data):  ` `        ``self``.data ``=` `data  ` `        ``self``.``next` `=` `None` ` `  `# The function removes duplicates  ` `# from a sorted list  ` `def` `removeDuplicates(head):  ` `     `  `    ``# Pointer to store the pointer  ` `    ``# of a node to be deleted to_free  ` `     `  `    ``# Do nothing if the list is empty  ` `    ``if` `(head ``=``=` `None``):  ` `        ``return` ` `  `    ``# Traverse the list till last node  ` `    ``if` `(head.``next` `!``=` `None``):  ` `         `  `        ``# Compare head node with next node  ` `        ``if` `(head.data ``=``=` `head.``next``.data):  ` `             `  `            ``# The sequence of steps is important.  ` `            ``# to_free pointer stores the next of head  ` `            ``# pointer which is to be deleted.  ` `            ``to_free ``=` `head.``next` `            ``head.``next` `=` `head.``next``.``next` `             `  `            ``# free(to_free)  ` `            ``removeDuplicates(head)  ` `         `  `        ``# This is tricky: only advance if no deletion ` `        ``else``:  ` `            ``removeDuplicates(head.``next``)  ` `         `  `    ``return` `head ` ` `  `# UTILITY FUNCTIONS  ` `# Function to insert a node at the  ` `# beginning of the linked list  ` `def` `push(head_ref, new_data):  ` `     `  `    ``# Allocate node  ` `    ``new_node ``=` `Node(new_data)  ` `             `  `    ``# Put in the data  ` `    ``new_node.data ``=` `new_data  ` `                 `  `    ``# Link the old list off the  ` `    ``# new node  ` `    ``new_node.``next` `=` `head_ref      ` `         `  `    ``# Move the head to point to the  ` `    ``# new node  ` `    ``head_ref ``=` `new_node ` `    ``return` `head_ref ` ` `  `# Function to print nodes in a given  ` `# linked list  ` `def` `printList(node):  ` `    ``while` `(node !``=` `None``):  ` `        ``print``(node.data, end ``=` `" "``)  ` `        ``node ``=` `node.``next` `     `  `# Driver code ` `if` `__name__``=``=``'__main__'``:  ` ` `  `    ``# Start with the empty list  ` `    ``head ``=` `None` `     `  `    ``# Let us create a sorted linked list ` `    ``# to test the functions  ` `    ``# Created linked list will be  ` `    ``# 11.11.11.13.13.20  ` `    ``head ``=` `push(head, ``20``)  ` `    ``head ``=` `push(head, ``13``)  ` `    ``head ``=` `push(head, ``13``)  ` `    ``head ``=` `push(head, ``11``)  ` `    ``head ``=` `push(head, ``11``)  ` `    ``head ``=` `push(head, ``11``)                                  ` ` `  `    ``print``(``"Linked list before duplicate removal "``, ` `           ``end ``=` `"")  ` `    ``printList(head)  ` ` `  `    ``# Remove duplicates from linked list  ` `    ``removeDuplicates(head)  ` ` `  `    ``print``(``"Linked list after duplicate removal "``, ` `           ``end ``=` `"")  ` `    ``printList(head)          ` `     `  `# This code is contributed by Srathore `

Output:

```Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20```

Another Approach: Create a pointer that will point towards the first occurrence of every element and another pointer temp which will iterate to every element and when the value of the previous pointer is not equal to the temp pointer, we will set the pointer of the previous pointer to the first occurrence of another node.

Below is the implementation of the above approach:

Python3

 `# Python3 program to remove duplicates  ` `# from a sorted linked list   ` `import` `math  ` `   `  `# Link list node   ` `class` `Node:   ` `     `  `    ``def` `__init__(``self``, data):         ` `        ``self``.data ``=` `data   ` `        ``self``.``next` `=` `None` `   `  `# The function removes duplicates   ` `# from the given linked list  ` `def` `removeDuplicates(head):   ` `     `  `    ``# Do nothing if the list consist of  ` `    ``# only one element or empty   ` `    ``if` `(head ``=``=` `None` `and`  `        ``head.``next` `=``=` `None``): ` `        ``return` `     `  `    ``# Construct a pointer  ` `    ``# pointing towards head ` `    ``current ``=` `head ` `     `  `    ``# Initialise a while loop till the  ` `    ``# second last node of the linkedlist  ` `    ``while` `(current.``next``): ` `         `  `        ``# If the data of current and next ` `        ``# node is equal we will skip the ` `        ``# node between them ` `        ``if` `current.data ``=``=` `current.``next``.data: ` `            ``current.``next` `=` `current.``next``.``next` `             `  `        ``# If the data of current and  ` `        ``# next node is different move ` `        ``# the pointer to the next node ` `        ``else``: ` `            ``current ``=` `current.``next` `     `  `    ``return` `   `  `# UTILITY FUNCTIONS   ` `# Function to insert a node at the   ` `# beginning of the linked list   ` `def` `push(head_ref, new_data):   ` `     `  `    ``# Allocate node   ` `    ``new_node ``=` `Node(new_data)   ` `               `  `    ``# Put in the data   ` `    ``new_node.data ``=` `new_data   ` `                   `  `    ``# Link the old list off  ` `    ``# the new node   ` `    ``new_node.``next` `=` `head_ref       ` `           `  `    ``# Move the head to point ` `    ``# to the new node   ` `    ``head_ref ``=` `new_node  ` `     `  `    ``return` `head_ref  ` `   `  `# Function to print nodes  ` `# in a given linked list   ` `def` `printList(node):   ` `     `  `    ``while` `(node !``=` `None``):   ` `        ``print``(node.data, end ``=` `" "``)   ` `        ``node ``=` `node.``next` `       `  `# Driver code  ` `if` `__name__``=``=``'__main__'``:   ` `   `  `    ``head ``=` `None`     `    ``head ``=` `push(head, ``20``)   ` `    ``head ``=` `push(head, ``13``)   ` `    ``head ``=` `push(head, ``13``)   ` `    ``head ``=` `push(head, ``11``)   ` `    ``head ``=` `push(head, ``11``)   ` `    ``head ``=` `push(head, ``11``)                                   ` `   `  `    ``print``(``"List before removal of "`   `          ``"duplicates "``, end ``=` `"")   ` `    ``printList(head)   ` `   `  `    ``removeDuplicates(head)   ` `   `  `    ``print``(``"List after removal of "`   `          ``"elements "``, end ``=` `"")             ` `    ``printList(head)           ` `# This code is contributed by MukulTomar `

Output:

```List before removal of duplicates
11 11 11 13 13 20
List after removal of elements
11 13 20 ```

Please refer complete article on Remove duplicates from a sorted linked list for more details!

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