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# Python 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.

## Python3

 `# Python3 program to swap elements of``# linked list by changing links` `# Linked List Node``class` `Node:   ``    ``def` `__init__(``self``, data):       ``        ``self``.data ``=` `data``        ``self``.``next` `=` `None` `# Create and Handle list``# operations``class` `LinkedList:``    ` `    ``def` `__init__(``self``):``        ` `        ``# Head of list``        ``self``.head ``=` `None` `    ``# Add data to list``    ``def` `addToList(``self``, data):       ``        ``newNode ``=` `Node(data)``        ``if` `self``.head ``is` `None``:``            ``self``.head ``=` `newNode``            ``return` `        ``last ``=` `self``.head``        ` `        ``while` `last.``next``:``            ``last ``=` `last.``next` `        ``last.``next` `=` `newNode` `    ``# Function to print nodes``    ``# in a given linked list``    ``def` `__str__(``self``):       ``        ``linkedListStr ``=` `""``        ``temp ``=` `self``.head``        ` `        ``while` `temp:``            ``linkedListStr ``=` `(linkedListStr ``+``                             ``str``(temp.data) ``+` `" "``)``            ``temp ``=` `temp.``next``            ` `        ``return` `linkedListStr` `    ``# 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``    ``def` `pairWiseSwap(``self``):` `        ``# If list is empty or with one``        ``# node``        ``if` `(``self``.head ``is` `None` `or``            ``self``.head.``next` `is` `None``):``            ``return` `        ``# Initialize previous and current``        ``# pointers``        ``prevNode ``=` `self``.head``        ``currNode ``=` `self``.head.``next` `        ``# Change head node``        ``self``.head ``=` `currNode` `        ``# Traverse the list``        ``while` `True``:``            ``nextNode ``=` `currNode.``next``            ` `            ``# Change next of current``            ``# node to previous node``            ``currNode.``next` `=` `prevNode ` `            ``# If next node is the last node``            ``if` `nextNode.``next` `is` `None``:``                ``prevNode.``next` `=` `nextNode``                ``break` `            ``# Change next of previous to``            ``# next of next``            ``prevNode.``next` `=` `nextNode.``next` `            ``# Update previous and current nodes``            ``prevNode ``=` `nextNode``            ``currNode ``=` `prevNode.``next` `# Driver Code``linkedList ``=` `LinkedList()``linkedList.addToList(``1``)``linkedList.addToList(``2``)``linkedList.addToList(``3``)``linkedList.addToList(``4``)``linkedList.addToList(``5``)``linkedList.addToList(``6``)``linkedList.addToList(``7``)` `print``(``"Linked list before calling"` `      ``"pairwiseSwap() "``, linkedList)``      ` `linkedList.pairWiseSwap()` `print``(``"Linked list after calling "``      ``"pairwiseSwap() "``, linkedList)``# This code is contributed by AmiyaRanjanRout`

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) because constant space has been used

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.

## Python3

 `# Python3 program to pairwise swap``# linked list using recursive method` `# Linked List Node``class` `Node:   ``    ``def` `__init__(``self``, data):       ``        ``self``.data ``=` `data``        ``self``.``next` `=` `None` `# Create and Handle list``# operations``class` `LinkedList:   ``    ``def` `__init__(``self``):``        ` `        ``# Head of list``        ``self``.head ``=` `None`  `    ``# Add data to list``    ``def` `addToList(``self``, data):       ``        ``newNode ``=` `Node(data)``        ` `        ``if` `self``.head ``is` `None``:``            ``self``.head ``=` `newNode``            ``return` `        ``last ``=` `self``.head``        ` `        ``while` `last.``next``:``            ``last ``=` `last.``next` `        ``last.``next` `=` `newNode` `    ``# Function to print nodes in``    ``# a given linked list``    ``def` `__str__(``self``):       ``        ``linkedListStr ``=` `""``        ``temp ``=` `self``.head``        ` `        ``while` `temp:``            ``linkedListStr ``=` `(linkedListStr ``+``                             ``str``(temp.data) ``+` `" "``)``            ``temp ``=` `temp.``next``        ``return` `linkedListStr` `    ``# 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``    ``def` `pairWiseSwap(``self``, node):` `        ``# If list is empty or with one node``        ``if` `node ``is` `None` `or` `node.``next` `is` `None``:``            ``return` `node` `        ``# Store head of list after``        ``# 2 nodes``        ``remaining ``=` `node.``next``.``next` `        ``# Change head``        ``newHead ``=` `node.``next` `        ``# Change next to second node``        ``node.``next``.``next` `=` `node` `        ``# Recur for remaining list and``        ``# change next of head``        ``node.``next` `=` `self``.pairWiseSwap(remaining)` `        ``# Return new head of modified list``        ``return` `newHead` `# Driver Code``linkedList ``=` `LinkedList()``linkedList.addToList(``1``)``linkedList.addToList(``2``)``linkedList.addToList(``3``)``linkedList.addToList(``4``)``linkedList.addToList(``5``)``linkedList.addToList(``6``)``linkedList.addToList(``7``)` `print``(``"Linked list before calling "``      ``"pairwiseSwap() "``, linkedList)``      ` `linkedList.head ``=` `linkedList.pairWiseSwap(``                  ``linkedList.head)``print``(``"Linked list after calling "``      ``"pairwiseSwap() "``, linkedList)``# This code is contributed by AmiyaRanjanRout`

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```

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

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