# Pairwise swap elements of a given linked list

Given a singly linked list, write a function to swap elements pairwise.

Input : 1->2->3->4->5->6->NULL
Output : 2->1->4->3->6->5->NULL

Input : 1->2->3->4->5->NULL
Output : 2->1->4->3->5->NULL

Input : 1->NULL
Output : 1->NULL

For example, if the linked list is 1->2->3->4->5 then the function should change it to 2->1->4->3->5, and if the linked list is then the function should change it to .

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

METHOD 1 (Iterative)
Start from the head node and traverse the list. While traversing swap data of each node with its next node’s data.

Below is the implementation of the above approach:

 `// C++ program to pairwise swap elements ` `// in a given linked list ` `#include ` `using` `namespace` `std; ` ` `  `/* A linked list node */` `class` `Node { ` `public``: ` `    ``int` `data; ` `    ``Node* next; ` `}; ` ` `  `/* Function to pairwise swap elements ` `of a linked list */` `void` `pairWiseSwap(Node* head) ` `{ ` `    ``Node* temp = head; ` ` `  `    ``/* Traverse further only if  ` `    ``there are at-least two nodes left */` `    ``while` `(temp != NULL && temp->next != NULL) { ` `        ``/* Swap data of node with  ` `           ``its next node's data */` `        ``swap(temp->data, ` `             ``temp->next->data); ` ` `  `        ``/* Move temp by 2 for the next pair */` `        ``temp = temp->next->next; ` `    ``} ` `} ` ` `  `/* Function to add a node at the  ` `   ``beginning of Linked 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() ` `{ ` `    ``Node* start = NULL; ` ` `  `    ``/* The constructed linked list is:  ` `    ``1->2->3->4->5 */` `    ``push(&start, 5); ` `    ``push(&start, 4); ` `    ``push(&start, 3); ` `    ``push(&start, 2); ` `    ``push(&start, 1); ` ` `  `    ``cout << ``"Linked list "` `         ``<< ``"before calling pairWiseSwap()\n"``; ` `    ``printList(start); ` ` `  `    ``pairWiseSwap(start); ` ` `  `    ``cout << ``"\nLinked list "` `         ``<< ``"after calling pairWiseSwap()\n"``; ` `    ``printList(start); ` ` `  `    ``return` `0; ` `} ` ` `  `// This code is contributed ` `// by rathbhupendra `

 `/* C program to pairwise swap elements in a given linked list */` `#include ` `#include ` ` `  `/* A linked list node */` `struct` `Node { ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `/*Function to swap two integers at addresses a and b */` `void` `swap(``int``* a, ``int``* b); ` ` `  `/* Function to pairwise swap elements of a linked list */` `void` `pairWiseSwap(``struct` `Node* head) ` `{ ` `    ``struct` `Node* temp = head; ` ` `  `    ``/* Traverse further only if there are at-least two nodes left */` `    ``while` `(temp != NULL && temp->next != NULL) { ` `        ``/* Swap data of node with its next node's data */` `        ``swap(&temp->data, &temp->next->data); ` ` `  `        ``/* Move temp by 2 for the next pair */` `        ``temp = temp->next->next; ` `    ``} ` `} ` ` `  `/* UTILITY FUNCTIONS */` `/* Function to swap two integers */` `void` `swap(``int``* a, ``int``* b) ` `{ ` `    ``int` `temp; ` `    ``temp = *a; ` `    ``*a = *b; ` `    ``*b = temp; ` `} ` ` `  `/* Function to add a node at the beginning of Linked 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 function */` `int` `main() ` `{ ` `    ``struct` `Node* start = NULL; ` ` `  `    ``/* The constructed linked list is:  ` `    ``1->2->3->4->5 */` `    ``push(&start, 5); ` `    ``push(&start, 4); ` `    ``push(&start, 3); ` `    ``push(&start, 2); ` `    ``push(&start, 1); ` ` `  `    ``printf``(``"Linked list before calling pairWiseSwap()\n"``); ` `    ``printList(start); ` ` `  `    ``pairWiseSwap(start); ` ` `  `    ``printf``(``"\nLinked list after calling pairWiseSwap()\n"``); ` `    ``printList(start); ` ` `  `    ``return` `0; ` `} `

 `// Java program to pairwise swap elements 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` `pairWiseSwap() ` `    ``{ ` `        ``Node temp = head; ` ` `  `        ``/* Traverse only till there are atleast 2 nodes left */` `        ``while` `(temp != ``null` `&& temp.next != ``null``) { ` ` `  `            ``/* Swap the data */` `            ``int` `k = temp.data; ` `            ``temp.data = temp.next.data; ` `            ``temp.next.data = k; ` `            ``temp = temp.next.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(); ` ` `  `        ``/* Created Linked List 1->2->3->4->5 */` `        ``llist.push(``5``); ` `        ``llist.push(``4``); ` `        ``llist.push(``3``); ` `        ``llist.push(``2``); ` `        ``llist.push(``1``); ` ` `  `        ``System.out.println(``"Linked List before calling pairWiseSwap() "``); ` `        ``llist.printList(); ` ` `  `        ``llist.pairWiseSwap(); ` ` `  `        ``System.out.println(``"Linked List after calling pairWiseSwap() "``); ` `        ``llist.printList(); ` `    ``} ` `} ` `/* This code is contributed by Rajat Mishra */`

 `# Python program to swap the elements of linked list pairwise ` ` `  `# 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 pairwise swap elements of a linked list ` `    ``def` `pairwiseSwap(``self``): ` `        ``temp ``=` `self``.head ` `        `  `        ``# There are no nodes in linked list ` `        ``if` `temp ``is` `None``: ` `            ``return`  `         `  `        ``# Traverse furthur only if there are at least two ` `        ``# left ` `        ``while``(temp ``is` `not` `None` `and` `temp.``next` `is` `not` `None``): ` `             `  `            ``# If both nodes are same, ` `            ``# no need to swap data ` `            ``if``(temp.data ``=``=` `temp.``next``.data): ` `                 `  `                ``# Move temp by 2 to the next pair ` `                ``temp ``=` `temp.``next``.``next` `            ``else``: ` `                 `  `                ``# Swap data of node with its next node's data ` `                ``temp.data, temp.``next``.data ``=` `temp.``next``.data, temp.data ` `                 `  `                ``# Move temp by 2 to the next pair ` `                ``temp ``=` `temp.``next``.``next` `        `  `    ``# 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 ` ` `  `    ``# Utility function to prit the linked LinkedList ` `    ``def` `printList(``self``): ` `        ``temp ``=` `self``.head ` `        ``while``(temp): ` `            ``print` `temp.data, ` `            ``temp ``=` `temp.``next` ` `  ` `  `# Driver program ` `llist ``=` `LinkedList() ` `llist.push(``5``) ` `llist.push(``4``) ` `llist.push(``3``) ` `llist.push(``2``) ` `llist.push(``1``) ` ` `  `print` `"Linked list before calling pairWiseSwap() "` `llist.printList() ` ` `  `llist.pairwiseSwap() ` ` `  `print`  `"\nLinked list after calling pairWiseSwap()"` `llist.printList() ` ` `  `# This code is contributed by Nikhil Kumar Singh(nickzuck_007) `

 `// C# program to pairwise swap elements of a linked list ` `using` `System; ` `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` `pairWiseSwap() ` `    ``{ ` `        ``Node temp = head; ` ` `  `        ``/* Traverse only till there are atleast 2 nodes left */` `        ``while` `(temp != ``null` `&& temp.next != ``null``) { ` ` `  `            ``/* Swap the data */` `            ``int` `k = temp.data; ` `            ``temp.data = temp.next.data; ` `            ``temp.next.data = k; ` `            ``temp = temp.next.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 program to test above functions */` `    ``public` `static` `void` `Main(String[] args) ` `    ``{ ` `        ``LinkedList llist = ``new` `LinkedList(); ` ` `  `        ``/* Created Linked List 1->2->3->4->5 */` `        ``llist.push(5); ` `        ``llist.push(4); ` `        ``llist.push(3); ` `        ``llist.push(2); ` `        ``llist.push(1); ` ` `  `        ``Console.WriteLine(``"Linked List before calling pairWiseSwap() "``); ` `        ``llist.printList(); ` ` `  `        ``llist.pairWiseSwap(); ` ` `  `        ``Console.WriteLine(``"Linked List after calling pairWiseSwap() "``); ` `        ``llist.printList(); ` `    ``} ` `} ` `// This code is contributed by Arnab Kundu `

Output:
```Linked List before calling pairWiseSwap()
1 2 3 4 5
2 1 4 3 5 ```

Time complexity: O(n)

METHOD 2 (Recursive)
If there are 2 or more than 2 nodes in Linked List then swap the first two nodes and recursively call for rest of the list.

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

 `/* Recursive function to pairwise swap elements ` `   ``of a linked list */` `void` `pairWiseSwap(``struct` `node* head) ` `{ ` `    ``/* There must be at-least two nodes in the list */` `    ``if` `(head != NULL && head->next != NULL) { ` ` `  `        ``/* Swap the node's data with data of next node */` `        ``swap(&head->data, &head->next->data); ` ` `  `        ``/* Call pairWiseSwap() for rest of the list */` `        ``pairWiseSwap(head->next->next); ` `    ``} ` `} `

Time complexity: O(n)

The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. See this for an implementation that changes links rather than swapping data.

Please write comments if you find any bug in above code/algorithm, or find other ways to solve the same problem.

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