# Pairwise swap elements of a given linked list by changing links

Given a singly linked list, write a function to swap elements pairwise. For example, if the linked list is 1->2->3->4->5->6->7 then the function should change it to 2->1->4->3->6->5->7, and if the linked list is 1->2->3->4->5->6 then the function should change it to 2->1->4->3->6->5

This problem has been discussed here. The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. So changing links is a better idea in general. Following is a C implementation that changes links instead of swapping data.

 `/* This program swaps the nodes of ` `linked list rather than swapping the ` `field from the nodes. ` `Imagine a case where a node contains` `many fields, there will be plenty ` `of unnecessary swap calls. */`   `#include ` `using` `namespace` `std;`   `/* A linked list node */` `class` `node {` `public``:` `    ``int` `data;` `    ``node* next;` `};`   `/* Function to pairwise swap elements` `of a linked list. It returns head of` `the modified list, so return value ` `of this node must be assigned */` `node* pairWiseSwap(node* head)` `{` `    ``// If linked list is empty or ` `    ``// there is only one node in list` `    ``if` `(head == NULL || head->next == NULL)` `        ``return` `head;` ` `  `    ``// Initialize previous and current pointers` `    ``node* prev = head;` `    ``node* curr = head->next;` ` `  `    ``head = curr; ``// Change head before proceeeding` ` `  `    ``// Traverse the list` `    ``while` `(``true``) {` `        ``node* next = curr->next;` `        ``curr->next = prev; ``// Change next of` `        ``// current as previous node` ` `  `        ``// If next NULL or next is the last node` `        ``if` `(next == NULL || next->next == NULL) {` `            ``prev->next = next;` `            ``break``;` `        ``}` ` `  `        ``// Change next of previous to next next` `        ``prev->next = next->next;` ` `  `        ``// Update previous and curr` `        ``prev = next;` `        ``curr = prev->next;` `    ``}` `    ``return` `head;` `}`   `/* Function to add a node at ` `the begining 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->6->7 */` `    ``push(&start, 7);` `    ``push(&start, 6);` `    ``push(&start, 5);` `    ``push(&start, 4);` `    ``push(&start, 3);` `    ``push(&start, 2);` `    ``push(&start, 1);`   `    ``cout << ``"Linked list before "` `      ``<< ``"calling pairWiseSwap() "``;` `    ``printList(start);`   `    ``start = pairWiseSwap(start); ``// NOTE THIS CHANGE`   `    ``cout << ``"\nLinked list after calling"` `      ``<< ``"pairWiseSwap() "``;` `    ``printList(start);`   `    ``return` `0;` `}`   `// This code is contributed by Manoj N`

 `/* This program swaps the nodes of linked list rather than swapping the` `field from the nodes.` `Imagine a case where a node contains many fields, there will be plenty` `of unnecessary swap calls. */`   `#include ` `#include ` `#include `   `/* A linked list node */` `struct` `Node {` `    ``int` `data;` `    ``struct` `Node* next;` `};`   `/* Function to pairwise swap elements of a linked list */` `void` `pairWiseSwap(``struct` `Node** head)` `{` `    ``// If linked list is empty or there is only one node in list` `    ``if` `(*head == NULL || (*head)->next == NULL)` `        ``return``;`   `    ``// Initialize previous and current pointers` `    ``struct` `Node* prev = *head;` `    ``struct` `Node* curr = (*head)->next;`   `    ``*head = curr; ``// Change head before proceeeding`   `    ``// Traverse the list` `    ``while` `(``true``) {` `        ``struct` `Node* next = curr->next;` `        ``curr->next = prev; ``// Change next of current as previous node`   `        ``// If next NULL or next is the last node` `        ``if` `(next == NULL || next->next == NULL) {` `            ``prev->next = next;` `            ``break``;` `        ``}`   `        ``// Change next of previous to next next` `        ``prev->next = next->next;`   `        ``// Update previous and curr` `        ``prev = next;` `        ``curr = prev->next;` `    ``}` `}`   `/* Function to add a node at the begining 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;` `    ``}` `}`   `/* Druver program to test above function */` `int` `main()` `{` `    ``struct` `Node* start = NULL;`   `    ``/* The constructed linked list is:` `     ``1->2->3->4->5->6->7 */` `    ``push(&start, 7);` `    ``push(&start, 6);` `    ``push(&start, 5);` `    ``push(&start, 4);` `    ``push(&start, 3);` `    ``push(&start, 2);` `    ``push(&start, 1);`   `    ``printf``(``"\n Linked list before calling  pairWiseSwap() "``);` `    ``printList(start);`   `    ``pairWiseSwap(&start);`   `    ``printf``(``"\n Linked list after calling  pairWiseSwap() "``);` `    ``printList(start);`   `    ``getchar``();` `    ``return` `0;` `}`

 `// Java program to swap elements of linked list by changing links`   `class` `LinkedList {`   `    ``static` `Node head;`   `    ``static` `class` `Node {`   `        ``int` `data;` `        ``Node next;`   `        ``Node(``int` `d)` `        ``{` `            ``data = d;` `            ``next = ``null``;` `        ``}` `    ``}`   `    ``/* Function to pairwise swap elements of a linked list */` `    ``Node pairWiseSwap(Node node)` `    ``{`   `        ``// If linked list is empty or there is only one node in list` `        ``if` `(node == ``null` `|| node.next == ``null``) {` `            ``return` `node;` `        ``}`   `        ``// Initialize previous and current pointers` `        ``Node prev = node;` `        ``Node curr = node.next;`   `        ``node = curr; ``// Change head before proceeeding`   `        ``// Traverse the list` `        ``while` `(``true``) {` `            ``Node next = curr.next;` `            ``curr.next = prev; ``// Change next of current as previous node`   `            ``// If next NULL or next is the last node` `            ``if` `(next == ``null` `|| next.next == ``null``) {` `                ``prev.next = next;` `                ``break``;` `            ``}`   `            ``// Change next of previous to next next` `            ``prev.next = next.next;`   `            ``// Update previous and curr` `            ``prev = next;` `            ``curr = prev.next;` `        ``}` `        ``return` `node;` `    ``}`   `    ``/* Function to print nodes in a given linked list */` `    ``void` `printList(Node node)` `    ``{` `        ``while` `(node != ``null``) {` `            ``System.out.print(node.data + ``" "``);` `            ``node = node.next;` `        ``}` `    ``}`   `    ``// Driver program to test above functions` `    ``public` `static` `void` `main(String[] args)` `    ``{`   `        ``/* The constructed linked list is:` `         ``1->2->3->4->5->6->7 */` `        ``LinkedList list = ``new` `LinkedList();` `        ``list.head = ``new` `Node(``1``);` `        ``list.head.next = ``new` `Node(``2``);` `        ``list.head.next.next = ``new` `Node(``3``);` `        ``list.head.next.next.next = ``new` `Node(``4``);` `        ``list.head.next.next.next.next = ``new` `Node(``5``);` `        ``list.head.next.next.next.next.next = ``new` `Node(``6``);` `        ``list.head.next.next.next.next.next.next = ``new` `Node(``7``);`   `        ``System.out.println(``"Linked list before calling pairwiseSwap() "``);` `        ``list.printList(head);` `        ``Node st = list.pairWiseSwap(head);` `        ``System.out.println(``""``);` `        ``System.out.println(``"Linked list after calling pairwiseSwap() "``);` `        ``list.printList(st);` `        ``System.out.println(``""``);` `    ``}` `}`   `// This code has been contributed by Mayank Jaiswal`

 `// C# program to swap elements of` `// linked list by changing links` `using` `System;`   `public` `class` `LinkedList {`   `    ``Node head;`   `    ``public` `class` `Node {`   `        ``public` `int` `data;` `        ``public` `Node next;`   `        ``public` `Node(``int` `d)` `        ``{` `            ``data = d;` `            ``next = ``null``;` `        ``}` `    ``}`   `    ``/* Function to pairwise swap ` `    ``elements of a linked list */` `    ``Node pairWiseSwap(Node node)` `    ``{`   `        ``// If linked list is empty or there` `        ``// is only one node in list` `        ``if` `(node == ``null` `|| node.next == ``null``) {` `            ``return` `node;` `        ``}`   `        ``// Initialize previous and current pointers` `        ``Node prev = node;` `        ``Node curr = node.next;`   `        ``// Change head before proceeeding` `        ``node = curr;`   `        ``// Traverse the list` `        ``while` `(``true``) {` `            ``Node next = curr.next;`   `            ``// Change next of current as previous node` `            ``curr.next = prev;`   `            ``// If next NULL or next is the last node` `            ``if` `(next == ``null` `|| next.next == ``null``) {` `                ``prev.next = next;` `                ``break``;` `            ``}`   `            ``// Change next of previous to next next` `            ``prev.next = next.next;`   `            ``// Update previous and curr` `            ``prev = next;` `            ``curr = prev.next;` `        ``}` `        ``return` `node;` `    ``}`   `    ``/* Function to print nodes ` `    ``in a given linked list */` `    ``void` `printList(Node node)` `    ``{` `        ``while` `(node != ``null``) {` `            ``Console.Write(node.data + ``" "``);` `            ``node = node.next;` `        ``}` `    ``}`   `    ``// Driver code` `    ``public` `static` `void` `Main(String[] args)` `    ``{`   `        ``/* The constructed linked list is: ` `        ``1->2->3->4->5->6->7 */` `        ``LinkedList list = ``new` `LinkedList();` `        ``list.head = ``new` `Node(1);` `        ``list.head.next = ``new` `Node(2);` `        ``list.head.next.next = ``new` `Node(3);` `        ``list.head.next.next.next = ``new` `Node(4);` `        ``list.head.next.next.next.next = ``new` `Node(5);` `        ``list.head.next.next.next.next.next = ``new` `Node(6);` `        ``list.head.next.next.next.next.next.next = ``new` `Node(7);`   `        ``Console.WriteLine(``"Linked list before calling pairwiseSwap() "``);` `        ``list.printList(list.head);` `        ``Node st = list.pairWiseSwap(list.head);` `        ``Console.WriteLine(``""``);` `        ``Console.WriteLine(``"Linked list after calling pairwiseSwap() "``);` `        ``list.printList(st);` `        ``Console.WriteLine(``""``);` `    ``}` `}`   `// This code contributed by Rajput-Ji`

Output:

``` Linked list before calling  pairWiseSwap() 1 2 3 4 5 6 7
Linked list after calling  pairWiseSwap() 2 1 4 3 6 5 7
```

Time Complexity: The time complexity of the above program is O(n) where n is the number of nodes in a given linked list. The while loop does a traversal of the given linked list.
Following is the recursive implementation of the same approach. We change first two nodes and recur for the remaining list. Thanks to geek and omer salem for suggesting this method.

 `/* This program swaps the nodes of linked list rather than swapping the ` `field from the nodes. ` `Imagine a case where a node contains many fields, there will be plenty ` `of unnecessary swap calls. */`   `#include ` `using` `namespace` `std;`   `/* A linked list node */` `class` `node {` `public``:` `    ``int` `data;` `    ``node* next;` `};`   `/* Function to pairwise swap elements of a linked list. ` `It returns head of the modified list, so return value ` `of this node must be assigned */` `node* pairWiseSwap(node* head)` `{` `    ``// Base Case: The list is empty or has only one node` `    ``if` `(head == NULL || head->next == NULL)` `        ``return` `head;`   `    ``// Store head of list after two nodes` `    ``node* remaing = head->next->next;`   `    ``// Change head` `    ``node* newhead = head->next;`   `    ``// Change next of second node` `    ``head->next->next = head;`   `    ``// Recur for remaining list and change next of head` `    ``head->next = pairWiseSwap(remaing);`   `    ``// Return new head of modified list` `    ``return` `newhead;` `}`   `/* Function to add a node at the begining 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;` `    ``}` `}`   `/* Druver program to test above function */` `int` `main()` `{` `    ``node* start = NULL;`   `    ``/* The constructed linked list is: ` `    ``1->2->3->4->5->6->7 */` `    ``push(&start, 7);` `    ``push(&start, 6);` `    ``push(&start, 5);` `    ``push(&start, 4);` `    ``push(&start, 3);` `    ``push(&start, 2);` `    ``push(&start, 1);`   `    ``cout << ``"Linked list before calling pairWiseSwap() "``;` `    ``printList(start);`   `    ``start = pairWiseSwap(start); ``// NOTE THIS CHANGE`   `    ``cout << ``"\nLinked list after calling pairWiseSwap() "``;` `    ``printList(start);`   `    ``return` `0;` `}`   `// This is code is contributed by rathbhupendra`

 `/* This program swaps the nodes of linked list rather than swapping the` `field from the nodes.` `Imagine a case where a node contains many fields, there will be plenty` `of unnecessary swap calls. */`   `#include ` `#include ` `#include `   `/* A linked list node */` `struct` `node {` `    ``int` `data;` `    ``struct` `node* next;` `};`   `/* Function to pairwise swap elements of a linked list.` `   ``It returns head of the modified list, so return value` `   ``of this node must be assigned */` `struct` `node* pairWiseSwap(``struct` `node* head)` `{` `    ``// Base Case: The list is empty or has only one node` `    ``if` `(head == NULL || head->next == NULL)` `        ``return` `head;`   `    ``// Store head of list after two nodes` `    ``struct` `node* remaing = head->next->next;`   `    ``// Change head` `    ``struct` `node* newhead = head->next;`   `    ``// Change next of second node` `    ``head->next->next = head;`   `    ``// Recur for remaining list and change next of head` `    ``head->next = pairWiseSwap(remaing);`   `    ``// Return new head of modified list` `    ``return` `newhead;` `}`   `/* Function to add a node at the begining 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;` `    ``}` `}`   `/* Druver program to test above function */` `int` `main()` `{` `    ``struct` `node* start = NULL;`   `    ``/* The constructed linked list is:` `     ``1->2->3->4->5->6->7 */` `    ``push(&start, 7);` `    ``push(&start, 6);` `    ``push(&start, 5);` `    ``push(&start, 4);` `    ``push(&start, 3);` `    ``push(&start, 2);` `    ``push(&start, 1);`   `    ``printf``(``"\n Linked list before calling  pairWiseSwap() "``);` `    ``printList(start);`   `    ``start = pairWiseSwap(start); ``// NOTE THIS CHANGE`   `    ``printf``(``"\n Linked list after calling  pairWiseSwap() "``);` `    ``printList(start);`   `    ``return` `0;` `}`

 `// Java program to swap elements of linked list by changing links`   `class` `LinkedList {`   `    ``static` `Node head;`   `    ``static` `class` `Node {`   `        ``int` `data;` `        ``Node next;`   `        ``Node(``int` `d)` `        ``{` `            ``data = d;` `            ``next = ``null``;` `        ``}` `    ``}`   `    ``/* Function to pairwise swap elements of a linked list.` `     ``It returns head of the modified list, so return value` `     ``of this node must be assigned */` `    ``Node pairWiseSwap(Node node)` `    ``{`   `        ``// Base Case: The list is empty or has only one node` `        ``if` `(node == ``null` `|| node.next == ``null``) {` `            ``return` `node;` `        ``}`   `        ``// Store head of list after two nodes` `        ``Node remaing = node.next.next;`   `        ``// Change head` `        ``Node newhead = node.next;`   `        ``// Change next of second node` `        ``node.next.next = node;`   `        ``// Recur for remaining list and change next of head` `        ``node.next = pairWiseSwap(remaing);`   `        ``// Return new head of modified list` `        ``return` `newhead;` `    ``}`   `    ``/* Function to print nodes in a given linked list */` `    ``void` `printList(Node node)` `    ``{` `        ``while` `(node != ``null``) {` `            ``System.out.print(node.data + ``" "``);` `            ``node = node.next;` `        ``}` `    ``}`   `    ``// Driver program to test above functions` `    ``public` `static` `void` `main(String[] args)` `    ``{`   `        ``/* The constructed linked list is:` `         ``1->2->3->4->5->6->7 */` `        ``LinkedList list = ``new` `LinkedList();` `        ``list.head = ``new` `Node(``1``);` `        ``list.head.next = ``new` `Node(``2``);` `        ``list.head.next.next = ``new` `Node(``3``);` `        ``list.head.next.next.next = ``new` `Node(``4``);` `        ``list.head.next.next.next.next = ``new` `Node(``5``);` `        ``list.head.next.next.next.next.next = ``new` `Node(``6``);` `        ``list.head.next.next.next.next.next.next = ``new` `Node(``7``);`   `        ``System.out.println(``"Linked list before calling pairwiseSwap() "``);` `        ``list.printList(head);` `        ``head = list.pairWiseSwap(head);` `        ``System.out.println(``""``);` `        ``System.out.println(``"Linked list after calling pairwiseSwap() "``);` `        ``list.printList(head);` `        ``System.out.println(``""``);` `    ``}` `}`

 `// C# program to swap elements` `// of linked list by changing links` `using` `System;`   `public` `class` `LinkedList {` `    ``Node head;`   `    ``class` `Node {` `        ``public` `int` `data;` `        ``public` `Node next;`   `        ``public` `Node(``int` `d)` `        ``{` `            ``data = d;` `            ``next = ``null``;` `        ``}` `    ``}`   `    ``/* Function to pairwise swap ` `        ``elements of a linked list.` `        ``It returns head of the modified` `        ``list, so return value of this` `        ``node must be assigned */` `    ``Node pairWiseSwap(Node node)` `    ``{`   `        ``// Base Case: The list is empty` `        ``// or has only one node` `        ``if` `(node == ``null` `|| node.next == ``null``) {` `            ``return` `node;` `        ``}`   `        ``// Store head of list after two nodes` `        ``Node remaing = node.next.next;`   `        ``// Change head` `        ``Node newhead = node.next;`   `        ``// Change next of second node` `        ``node.next.next = node;`   `        ``// Recur for remaining list` `        ``// and change next of head` `        ``node.next = pairWiseSwap(remaing);`   `        ``// Return new head of modified list` `        ``return` `newhead;` `    ``}`   `    ``/* Function to print nodes in a given linked list */` `    ``void` `printList(Node node)` `    ``{` `        ``while` `(node != ``null``) {` `            ``Console.Write(node.data + ``" "``);` `            ``node = node.next;` `        ``}` `    ``}`   `    ``// Driver program to test above functions` `    ``public` `static` `void` `Main()` `    ``{`   `        ``/* The constructed linked list is:` `        ``1->2->3->4->5->6->7 */` `        ``LinkedList list = ``new` `LinkedList();` `        ``list.head = ``new` `Node(1);` `        ``list.head.next = ``new` `Node(2);` `        ``list.head.next.next = ``new` `Node(3);` `        ``list.head.next.next.next = ``new` `Node(4);` `        ``list.head.next.next.next.next = ``new` `Node(5);` `        ``list.head.next.next.next.next.next = ``new` `Node(6);` `        ``list.head.next.next.next.next.next.next = ``new` `Node(7);`   `        ``Console.WriteLine(``"Linked list before calling pairwiseSwap() "``);` `        ``list.printList(list.head);` `        ``list.head = list.pairWiseSwap(list.head);` `        ``Console.WriteLine(``""``);` `        ``Console.WriteLine(``"Linked list after calling pairwiseSwap() "``);` `        ``list.printList(list.head);` `        ``Console.WriteLine(``""``);` `    ``}` `}`   `// This code is contributed by PrinciRaj1992`

Output:

``` Linked list before calling  pairWiseSwap() 1 2 3 4 5 6 7
Linked list after calling  pairWiseSwap() 2 1 4 3 6 5 7
```

Pairwise swap adjacent nodes of a linked list by changing pointers | Set 2