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# C Program For Pairwise Swapping 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 the implementation that changes links instead of swapping data.

## C

 `/* 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;` `    ``// Change head before proceeding``    ``*head = curr;` `    ``// Traverse the list``    ``while` `(``true``)``    ``{``        ``struct` `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;``    ``}``}` `/* 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 of 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 code``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``("``    ``Linked list before calling  pairWiseSwap() ");``    ``printList(start);` `    ``pairWiseSwap(&start);` `    ``printf``("``    ``Linked list after calling  pairWiseSwap() ");``    ``printList(start);` `    ``getchar``();``    ``return` `0;``}`

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.

Auxiliary Space: O(1)

Following is the recursive implementation of the same approach. We change the first two nodes and recur for the remaining list. Thanks to geek and omer salem for suggesting this method.

## C

 `/* 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* remaining =``           ``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(remaining);` `    ``// Return new head of modified list``    ``return` `newhead;``}` `/* 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 of 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 code``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``("``    ``Linked list before calling  pairWiseSwap() ");``    ``printList(start);` `    ``// NOTE THIS CHANGE``    ``start = pairWiseSwap(start);` `    ``printf``("``    ``Linked list after calling  pairWiseSwap() ");``    ``printList(start);` `    ``return` `0;``}`

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: O(n)

Auxiliary Space: O(n)

Please refer complete article on Pairwise swap elements of a given linked list by changing links for more details!

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