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Iteratively Reverse a linked list using only 2 pointers (An Interesting Method)
• Difficulty Level : Medium
• Last Updated : 30 Dec, 2020

Given pointer to the head node of a linked list, the task is to reverse the linked list.

Examples:

```Input : Head of following linked list
1->2->3->4->NULL
Output : Linked list should be changed to,
4->3->2->1->NULL

Input : Head of following linked list
1->2->3->4->5->NULL
Output : Linked list should be changed to,
5->4->3->2->1->NULL```

We have seen how to reverse a linked list in article Reverse a linked list. In iterative method we had used 3 pointers prev, cur and next. Below is an interesting approach that uses only two pointers. The idea is to use XOR to swap pointers.

## C++

 `// C++ program to reverse a linked list using two pointers.``#include ``using` `namespace` `std;``typedef` `uintptr_t` `ut;` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `/* Function to reverse the linked list using 2 pointers */``void` `reverse(``struct` `Node** head_ref)``{``    ``struct` `Node* prev = NULL;``    ``struct` `Node* current = *head_ref;` `    ``// at last prev points to new head``    ``while` `(current != NULL) {``        ``// This expression evaluates from left to right``        ``// current->next = prev, changes the link fron``        ``// next to prev node``        ``// prev = current, moves prev to current node for``        ``// next reversal of node``        ``// This example of list will clear it more 1->2->3->4``        ``// initially prev = 1, current = 2``        ``// Final expression will be current = 1^2^3^2^1,``        ``// as we know that bitwise XOR of two same``        ``// numbers will always be 0 i.e; 1^1 = 2^2 = 0``        ``// After the evaluation of expression current = 3 that``        ``// means it has been moved by one node from its``        ``// previous position``        ``current = (``struct` `Node*)((ut)prev ^ (ut)current ^ (ut)(current->next) ^ (ut)(current->next = prev) ^ (ut)(prev = current));``    ``}` `    ``*head_ref = prev;``}` `/* Function to push a node */``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 linked list */``void` `printList(``struct` `Node* head)``{``    ``struct` `Node* temp = head;``    ``while` `(temp != NULL) {``        ``printf``(``"%d  "``, temp->data);``        ``temp = temp->next;``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 85);` `    ``printf``(``"Given linked list\n"``);``    ``printList(head);``    ``reverse(&head);``    ``printf``(``"\nReversed Linked list \n"``);``    ``printList(head);``    ``return` `0;``}`

## Java

 `// Java program to reverse a linked``// list using two pointers.``import` `java.util.*;` `class` `Main{``    ` `// Link list node``static` `class` `Node``{``    ``int` `data;``    ``Node next;``};` `static` `Node head_ref = ``null``;`` ` `// Function to reverse the linked``// list using 2 pointers``static` `void` `reverse()``{``    ``Node prev = ``null``;``    ``Node current = head_ref;``    ` `    ``// At last prev points to new head``    ``while` `(current != ``null``)``    ``{``        ` `        ``// This expression evaluates from left to right``        ``// current.next = prev, changes the link fron``        ``// next to prev node``        ``// prev = current, moves prev to current node for``        ``// next reversal of node``        ``// This example of list will clear it more 1.2.3.4``        ``// initially prev = 1, current = 2``        ``// Final expression will be current = 1^2^3^2^1,``        ``// as we know that bitwise XOR of two same``        ``// numbers will always be 0 i.e; 1^1 = 2^2 = 0``        ``// After the evaluation of expression current = 3 that``        ``// means it has been moved by one node from its``        ``// previous position``        ``Node next = current.next;``        ``current.next = prev;``        ``prev = current;``        ``current = next;``    ``}``    ``head_ref = prev;``}`` ` `// Function to push a node``static` `void` `push(``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 linked list``static` `void` `printList()``{``    ``Node temp = head_ref;``    ``while` `(temp != ``null``)``    ``{``        ``System.out.print(temp.data + ``" "``);``        ``temp = temp.next;``    ``}``}`` ` `// Driver code``public` `static` `void` `main(String []args)``{``    ``push(``20``);``    ``push(``4``);``    ``push(``15``);``    ``push(``85``);`` ` `    ``System.out.print(``"Given linked list\n"``);``    ``printList();``    ``reverse();``    ``System.out.print(``"\nReversed Linked list \n"``);``    ``printList();``}``}` `// This code is contributed by rutvik_56`

## Python

 `# Iteratively Reverse a linked list using only 2 pointers (An Interesting Method)``# Python program to reverse a linked list``# Link list node ``# 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 reverse the linked list``    ``def` `reverse(``self``):``        ``prev ``=` `None``        ``current ``=` `self``.head``    ``# Described here https://www.geeksforgeeks.org/``    ``# how-to-swap-two-variables-in-one-line / while(current is not None):``            ``# This expression evaluates from left to right``            ``# current->next = prev, changes the link fron``            ``# next to prev node``            ``# prev = current, moves prev to current node for``            ``# next reversal of node``            ``# This example of list will clear it more 1->2``            ``# initially prev = 1, current = 2``            ``# Final expression will be current = 1, prev = 2``            ``next``, current.``next` `=` `current.``next``, prev``            ``prev, current ``=` `current, ``next``        ``self``.head ``=` `prev``         ` `    ``# Function to push a new node``    ``def` `push(``self``, new_data):``        ``# allocate node and put in the data``        ``new_node ``=` `node(new_data)``        ``# link the old list off the new node``        ``new_node.``next` `=` `self``.head``        ``# move the head to point to the new node``        ``self``.head ``=` `new_node`` ` `    ``# Function to print the linked list``    ``def` `printList(``self``):``        ``temp ``=` `self``.head``        ``while``(temp):``            ``print` `temp.data,``            ``temp ``=` `temp.``next`` ` ` ` `# Driver program to test above functions``llist ``=` `LinkedList()``llist.push(``20``)``llist.push(``4``)``llist.push(``15``)``llist.push(``85``)`` ` `print` `"Given Linked List"``llist.printList()``llist.reverse()``print` `"\nReversed Linked List"``llist.printList()` `# This code is contributed by Afzal Ansari`

## C#

 `// C# program to reverse a linked``// list using two pointers.``using` `System;``using` `System.Collections;``using` `System.Collections.Generic;``class` `GFG``{` `  ``// Link list node``  ``class` `Node``  ``{``    ``public` `int` `data;``    ``public` `Node next;``  ``};` `  ``static` `Node head_ref = ``null``;` `  ``// Function to reverse the linked``  ``// list using 2 pointers``  ``static` `void` `reverse()``  ``{``    ``Node prev = ``null``;``    ``Node current = head_ref;` `    ``// At last prev points to new head``    ``while` `(current != ``null``)``    ``{` `      ``// This expression evaluates from left to right``      ``// current.next = prev, changes the link fron``      ``// next to prev node``      ``// prev = current, moves prev to current node for``      ``// next reversal of node``      ``// This example of list will clear it more 1.2.3.4``      ``// initially prev = 1, current = 2``      ``// Final expression will be current = 1^2^3^2^1,``      ``// as we know that bitwise XOR of two same``      ``// numbers will always be 0 i.e; 1^1 = 2^2 = 0``      ``// After the evaluation of expression current = 3 that``      ``// means it has been moved by one node from its``      ``// previous position``      ``Node next = current.next;``      ``current.next = prev;``      ``prev = current;``      ``current = next;``    ``}``    ``head_ref = prev;``  ``}` `  ``// Function to push a node``  ``static` `void` `push(``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 linked list``  ``static` `void` `printList()``  ``{``    ``Node temp = head_ref;``    ``while` `(temp != ``null``)``    ``{``      ``Console.Write(temp.data + ``" "``);``      ``temp = temp.next;``    ``}``  ``}` `  ``// Driver code``  ``public` `static` `void` `Main(``string` `[]args)``  ``{``    ``push(20);``    ``push(4);``    ``push(15);``    ``push(85);` `    ``Console.Write(``"Given linked list\n"``);``    ``printList();``    ``reverse();``    ``Console.Write(``"\nReversed Linked list \n"``);``    ``printList();``  ``}``}` `// This code is contributed by pratham76`
Output:
```Given linked list
85  15  4  20
Reversed Linked list
20  4  15  85```

Time Complexity: O(n)
Reference :
http://discuss.joelonsoftware.com/default.asp?interview.11.564944.16
Alternate Solution :

## C++

 `// C++ program to reverse a linked list using two pointers.``#include ``using` `namespace` `std;``typedef` `uintptr_t` `ut;` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `/* Function to reverse the linked list using 2 pointers */``void` `reverse(``struct` `Node** head_ref)``{``    ``struct` `Node* current = *head_ref;``    ``struct` `Node* next;``    ``while` `(current->next != NULL) {``        ``next = current->next;``        ``current->next = next->next;``        ``next->next = (*head_ref);``        ``*head_ref = next;``    ``}``}` `/* Function to push a node */``void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``struct` `Node* new_node = ``new` `Node;``    ``new_node->data = new_data;``    ``new_node->next = (*head_ref);``    ``(*head_ref) = new_node;``}` `/* Function to print linked list */``void` `printList(``struct` `Node* head)``{``    ``struct` `Node* temp = head;``    ``while` `(temp != NULL) {``        ``printf``(``"%d  "``, temp->data);``        ``temp = temp->next;``    ``}``}` `/* Driver program to test above function*/``int` `main()``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;` `    ``push(&head, 20);``    ``push(&head, 4);``    ``push(&head, 15);``    ``push(&head, 85);` `    ``printf``(``"Given linked list\n"``);``    ``printList(head);``    ``reverse(&head);``    ``printf``(``"\nReversed Linked list \n"``);``    ``printList(head);``    ``return` `0;``}`

## Python3

 `# Python3 program to reverse a linked list using two pointers.` `# A linked list node``class` `Node :``    ``def` `__init__(``self``):``        ``self``.data ``=` `0``        ``self``.``next` `=` `None` `# Function to reverse the linked list using 2 pointers``def` `reverse(head_ref):` `    ``current ``=` `head_ref``    ``next``=` `None``    ``while` `(current.``next` `!``=` `None``) :``        ``next` `=` `current.``next``        ``current.``next` `=` `next``.``next``        ``next``.``next` `=` `(head_ref)``        ``head_ref ``=` `next``    ` `    ``return` `head_ref` `# Function to push a node``def` `push( head_ref, new_data):` `    ``new_node ``=` `Node()``    ``new_node.data ``=` `new_data``    ``new_node.``next` `=` `(head_ref)``    ``(head_ref) ``=` `new_node``    ``return` `head_ref` `# Function to print linked list``def` `printList( head):` `    ``temp ``=` `head``    ``while` `(temp !``=` `None``) :``        ``print``( temp.data, end``=``" "``)``        ``temp ``=` `temp.``next``    ` `# Driver code` `# Start with the empty list``head ``=` `None` `head ``=` `push(head, ``20``)``head ``=` `push(head, ``4``)``head ``=` `push(head, ``15``)``head ``=` `push(head, ``85``)` `print``(``"Given linked list"``)``printList(head)``head ``=` `reverse(head)``print``(``"\nReversed Linked list "``)``printList(head)` `# This code is contributed by Arnab Kundu`
Output:
```Given linked list
85  15  4  20
Reversed Linked list
20  4  15  85```

Thanks to Abhay Yadav for suggesting this approach.
This article is contributed by Shashank Mishra ( Gullu ). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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