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Delete alternate nodes of a Linked List
• Difficulty Level : Easy
• Last Updated : 19 May, 2021

Given a Singly Linked List, starting from the second node delete all alternate nodes of it. For example, if the given linked list is 1->2->3->4->5 then your function should convert it to 1->3->5, and if the given linked list is 1->2->3->4 then convert it to 1->3.

Method 1 (Iterative)
Keep track of previous of the node to be deleted. First, change the next link of the previous node and then free the memory allocated for the node.

## C++

 `// C++ program to remove alternate``// nodes of a linked list``#include ``using` `namespace` `std;` `/* A linked list node */``class` `Node``{``    ``public``:``    ``int` `data;``    ``Node *next;``};` `/* deletes alternate nodes``of a list starting with head */``void` `deleteAlt(Node *head)``{``    ``if` `(head == NULL)``        ``return``;` `    ``/* Initialize prev and node to be deleted */``    ``Node *prev = head;``    ``Node *node = head->next;` `    ``while` `(prev != NULL && node != NULL)``    ``{``        ``/* Change next link of previous node */``        ``prev->next = node->next;` `        ``/* Free memory */``        ``free``(node);` `        ``/* Update prev and node */``        ``prev = prev->next;``        ``if` `(prev != NULL)``            ``node = prev->next;``    ``}``}` `/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */``/* Given a reference (pointer to pointer) to the head``of a list and an int, push a new node on the front``of the list. */``void` `push(Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``Node* new_node = ``new` `Node();` `    ``/* 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;``}` `/* Function to print nodes in a given linked list */``void` `printList(Node *node)``{``    ``while` `(node != NULL)``    ``{``        ``cout<< node->data<<``" "``;``        ``node = node->next;``    ``}``}` `/* Driver code */``int` `main()``{``    ``/* Start with the empty list */``    ``Node* head = NULL;` `    ``/* Using push() to construct below list``    ``1->2->3->4->5 */``    ``push(&head, 5);``    ``push(&head, 4);``    ``push(&head, 3);``    ``push(&head, 2);``    ``push(&head, 1);` `    ``cout<<``"List before calling deleteAlt() \n"``;``    ``printList(head);` `    ``deleteAlt(head);` `    ``cout<<``"\nList after calling deleteAlt() \n"``;``    ``printList(head);` `    ``return` `0;``}` `// This code is contributed by rathbhupendra`

## C

 `// C program to remove alternate nodes of a linked list``#include``#include` `/* A linked list node */``struct` `Node``{``    ``int` `data;``    ``struct` `Node *next;``};` `/* deletes alternate nodes of a list starting with head */``void` `deleteAlt(``struct` `Node *head)``{``    ``if` `(head == NULL)``        ``return``;` `    ``/* Initialize prev and node to be deleted */``    ``struct` `Node *prev = head;``    ``struct` `Node *node = head->next;` `    ``while` `(prev != NULL && node != NULL)``    ``{``        ``/* Change next link of previous node */``        ``prev->next = node->next;` `        ``/* Free memory */``        ``free``(node);` `        ``/* Update prev and node */``        ``prev = prev->next;``        ``if` `(prev != NULL)``            ``node = prev->next;``    ``}``}` `/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */``/* Given a reference (pointer to pointer) to the head``  ``of a list and an int, push a new node on the front``  ``of the list. */``void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node =``        ``(``struct` `Node*) ``malloc``(``sizeof``(``struct` `Node));` `    ``/* 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;``}` `/* Function to print nodes in a given linked list */``void` `printList(``struct` `Node *node)``{``    ``while` `(node != NULL)``    ``{``        ``printf``(``"%d "``, node->data);``        ``node = node->next;``    ``}``}` `/* Driver program to test above functions */``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``/* Using push() to construct below list``      ``1->2->3->4->5  */``    ``push(&head, 5);``    ``push(&head, 4);``    ``push(&head, 3);``    ``push(&head, 2);``    ``push(&head, 1);` `    ``printf``(``"\nList before calling deleteAlt() \n"``);``    ``printList(head);` `    ``deleteAlt(head);` `    ``printf``(``"\nList after calling deleteAlt() \n"``);``    ``printList(head);` `    ``return` `0;``}`

## Java

 `// Java program to delete alternate nodes of a linked list``class` `LinkedList``{``    ``Node head;  ``// head of list`` ` `    ``/* Linked list Node*/``    ``class` `Node``    ``{``        ``int` `data;``        ``Node next;``        ``Node(``int` `d) {data = d; next = ``null``; }``    ``}` `    ``void` `deleteAlt()``    ``{``       ``if` `(head == ``null``)``          ``return``;` `       ``Node prev = head;``       ``Node now = head.next;` `       ``while` `(prev != ``null` `&& now != ``null``)``       ``{          ``           ``/* Change next link of previous node */``           ``prev.next = now.next;` `           ``/* Free node */``           ``now = ``null``;` `           ``/*Update prev and now */``           ``prev = prev.next;``           ``if` `(prev != ``null``)``              ``now = prev.next;``       ``}``    ``}                ` `                    ` `    ``/* Utility functions */` `    ``/* Inserts a new Node at front of the list. */``    ``public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                  ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);`` ` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;`` ` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``/* Function to print linked list */``    ``void` `printList()``    ``{``        ``Node temp = head;``        ``while``(temp != ``null``)``        ``{``           ``System.out.print(temp.data+``" "``);``           ``temp = temp.next;``        ``} ``        ``System.out.println();``    ``}` `     ``/* Driver program to test above functions */``    ``public` `static` `void` `main(String args[])``    ``{``        ``LinkedList llist = ``new` `LinkedList();``        ` `        ``/* Constructed Linked List is 1->2->3->4->5->null */``        ``llist.push(``5``);``        ``llist.push(``4``);``        ``llist.push(``3``);``        ``llist.push(``2``);``        ``llist.push(``1``);``        ` `        ``System.out.println(``"Linked List before calling deleteAlt() "``);``        ``llist.printList();``        ` `        ``llist.deleteAlt();``        ` `        ``System.out.println(``"Linked List after calling deleteAlt() "``);``        ``llist.printList();``    ``}``}``/* This code is contributed by Rajat Mishra */`

## Python3

 `# Python3 program to remove alternate``# nodes of a linked list``import` `math` `# A linked list node``class` `Node:``    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.``next` `=` `None``        ` `# deletes alternate nodes``# of a list starting with head``def` `deleteAlt(head):``    ``if` `(head ``=``=` `None``):``        ``return` `    ``# Initialize prev and node to be deleted``    ``prev ``=` `head``    ``now ``=` `head.``next` `    ``while` `(prev !``=` `None` `and` `now !``=` `None``):``        ` `        ``# Change next link of previous node``        ``prev.``next` `=` `now.``next` `        ``# Free memory``        ``now ``=` `None` `        ``# Update prev and node``        ``prev ``=` `prev.``next``        ``if` `(prev !``=` `None``):``            ``now ``=` `prev.``next``    ` `# UTILITY FUNCTIONS TO TEST fun1() and fun2()``# Given a reference (poer to poer) to the head``# of a list and an , push a new node on the front``# of the 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 po to the new node``    ``head_ref ``=` `new_node``    ``return` `head_ref` `# Function to print nodes in a given linked list``def` `prList(node):``    ``while` `(node !``=` `None``):``        ``print``(node.data, end ``=` `" "``)``        ``node ``=` `node.``next``    ` `# Driver code``if` `__name__``=``=``'__main__'``:``    ` `    ``# Start with the empty list``    ``head ``=` `None` `    ``# Using head=push() to construct below list``    ``# 1.2.3.4.5``    ``head ``=` `push(head, ``5``)``    ``head ``=` `push(head, ``4``)``    ``head ``=` `push(head, ``3``)``    ``head ``=` `push(head, ``2``)``    ``head ``=` `push(head, ``1``)` `    ``print``(``"List before calling deleteAlt() "``)``    ``prList(head)` `    ``deleteAlt(head)` `    ``print``(``"\nList after calling deleteAlt() "``)``    ``prList(head)` `# This code is contributed by Srathore`

## C#

 `// C# program to delete alternate``// nodes of a linked list``using` `System;` `public` `class` `LinkedList``{``    ``Node head; ``// head of list` `    ``/* Linked list Node*/``    ``public` `class` `Node``    ``{``        ``public` `int` `data;``        ``public` `Node next;``        ``public` `Node(``int` `d)``        ``{``            ``data = d; next = ``null``;``            ` `        ``}``    ``}` `    ``void` `deleteAlt()``    ``{``        ``if` `(head == ``null``)``            ``return``;` `        ``Node prev = head;``        ``Node now = head.next;` `        ``while` `(prev != ``null` `&& now != ``null``)``        ``{        ``            ``/* Change next link of previous node */``            ``prev.next = now.next;` `            ``/* Free node */``            ``now = ``null``;` `            ``/*Update prev and now */``            ``prev = prev.next;``            ``if` `(prev != ``null``)``                ``now = prev.next;``        ``}``    ``}                ` `                    ` `    ``/* Utility functions */` `    ``/* Inserts a new Node at front of the list. */``    ``public` `void` `push(``int` `new_data)``    ``{``        ``/* 1 & 2: Allocate the Node &``                ``Put in the data*/``        ``Node new_node = ``new` `Node(new_data);` `        ``/* 3. Make next of new Node as head */``        ``new_node.next = head;` `        ``/* 4. Move the head to point to new Node */``        ``head = new_node;``    ``}` `    ``/* Function to print linked list */``    ``void` `printList()``    ``{``        ``Node temp = head;``        ``while``(temp != ``null``)``        ``{``        ``Console.Write(temp.data+``" "``);``        ``temp = temp.next;``        ``}``        ``Console.WriteLine();``    ``}` `    ``/* Driver code*/``    ``public` `static` `void` `Main(String []args)``    ``{``        ``LinkedList llist = ``new` `LinkedList();``        ` `        ``/* Constructed Linked List is``        ``1->2->3->4->5->null */``        ``llist.push(5);``        ``llist.push(4);``        ``llist.push(3);``        ``llist.push(2);``        ``llist.push(1);``        ` `        ``Console.WriteLine(``"Linked List before"` `+``                            ``"calling deleteAlt() "``);``        ``llist.printList();``        ` `        ``llist.deleteAlt();``        ` `        ``Console.WriteLine(``"Linked List after"` `+``                            ``"calling deleteAlt() "``);``        ``llist.printList();``    ``}``}` `// This code has been contributed``// by 29AjayKumar`

## Javascript

 ``

Output:

```List before calling deleteAlt()
1 2 3 4 5
List after calling deleteAlt()
1 3 5 ```

Time Complexity: O(n) where n is the number of nodes in the given Linked List.
Method 2 (Recursive)
Recursive code uses the same approach as method 1. The recursive code is simple and short but causes O(n) recursive function calls for a linked list of size n.

## C++

 `/* deletes alternate nodes of a list starting with head */``void` `deleteAlt(Node *head)``{``    ``if` `(head == NULL)``        ``return``;` `    ``Node *node = head->next;` `    ``if` `(node == NULL)``        ``return``;` `    ``/* Change the next link of head */``    ``head->next = node->next;` `    ``/* free memory allocated for node */``    ``free``(node);` `    ``/* Recursively call for the new next of head */``    ``deleteAlt(head->next);``}` `// This code is contributed by rathbhupendra`

## C

 `/* deletes alternate nodes of a list starting with head */``void` `deleteAlt(``struct` `Node *head)``{``    ``if` `(head == NULL)``        ``return``;` `    ``struct` `Node *node = head->next;` `    ``if` `(node == NULL)``        ``return``;` `    ``/* Change the next link of head */``    ``head->next = node->next;` `    ``/* free memory allocated for node */``    ``free``(node);` `    ``/* Recursively call for the new next of head */``    ``deleteAlt(head->next);``}`

## Java

 `/* deletes alternate nodes of a list``starting with head */``static` `Node deleteAlt(Node head)``{``    ``if` `(head == ``null``)``        ``return``;` `    ``Node node = head.next;` `    ``if` `(node == ``null``)``        ``return``;` `    ``/* Change the next link of head */``    ``head.next = node.next;`  `    ``/* Recursively call for the new next of head */``    ``head.next = deleteAlt(head.next);``}` `// This code is contributed by Arnab Kundu`

## Python3

 `# deletes alternate nodes of a list starting with head``def` `deleteAlt(head):``    ``if` `(head ``=``=` `None``):``        ``return` `    ``node ``=` `head.``next` `    ``if` `(node ``=``=` `None``):``        ``return` `    ``# Change the next link of head``    ``head.``next` `=` `node.``next` `    ``# free memory allocated for node``    ``#free(node)` `    ``# Recursively call for the new next of head``    ``deleteAlt(head.``next``)` `# This code is contributed by Srathore`

## C#

 `/* deletes alternate nodes of a list``starting with head */``static` `Node deleteAlt(Node head)``{``    ``if` `(head == ``null``)``        ``return``;` `    ``Node node = head.next;` `    ``if` `(node == ``null``)``        ``return``;` `    ``/* Change the next link of head */``    ``head.next = node.next;`  `    ``/* Recursively call for the new next of head */``    ``head.next = deleteAlt(head.next);``}` `// This code is contributed by Arnab Kundu`

## Javascript

 ``

Time Complexity: O(n)

Please write comments if you find the above code/algorithm incorrect, or find better ways to solve the same problem.

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