# Java Program For Pairwise Swapping Elements Of A Given Linked List By Changing Links

Last Updated : 30 Mar, 2022

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.

## Java

 `// 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;`   `        ``// Change head before proceeding` `        ``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``) ` `        ``{` `            ``System.out.print(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``);`   `        ``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 is contributed by Mayank Jaiswal`

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.

## Java

 `// 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 remaining = 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(remaining);`   `        ``// 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 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``);`   `        ``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(``""``);` `    ``}` `}`

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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