Pairwise swap elements of a given linked list by changing links

03 Jul, 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. 
Implementation:
C++
C
Java
Python3
C#
Javascript
/* This program swaps the nodes of linked list rather than swapping the field from the nodes. Imagine a case where a node containsmany fields, there will be plenty of unnecessary swap calls. */  #include <bits/stdc++.h>using namespace std;  /* A linked list node */class node {public:    int data;    node* next;};  /* Function to pairwise swap elementsof a linked list. It returns head ofthe 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 proceeding       // 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 of next        prev->next = next->next;           // Update previous and curr        prev = next;        curr = prev->next;    }    return head;}  /* 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->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 thefield from the nodes.Imagine a case where a node contains many fields, there will be plentyof unnecessary swap calls. */  #include <stdbool.h>#include <stdio.h>#include <stdlib.h>  /* 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 proceeding      // 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 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->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 proceeding          // 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
# Python3 program to swap elements of# linked list by changing links  # Linked List Nodeclass Node:          def __init__(self, data):                  self.data = data        self.next = None  # Create and Handle list operationsclass 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 CodelinkedList = 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
// C# program to swap elements of// linked list by changing linksusing 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 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 of 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
<script>// javascript program to swap elements of linked list by changing linksvar head;       class Node {            constructor(val) {                this.data = val;                this.next = null;            }        }             /* Function to pairwise swap elements of a linked list */    function pairWiseSwap(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        var prev = node;        var curr = node.next;          node = curr; // Change head before proceeding          // Traverse the list        while (true) {        var 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 */    function printList(node) {        while (node != null) {            document.write(node.data + " ");            node = node.next;        }    }      // Driver program to test above functions                /*         * The constructed linked list is: 1->2->3->4->5->6->7         */        head = new Node(1);        head.next = new Node(2);        head.next.next = new Node(3);        head.next.next.next = new Node(4);        head.next.next.next.next = new Node(5);        head.next.next.next.next.next = new Node(6);        head.next.next.next.next.next.next = new Node(7);          document.write("Linked list before calling pairwiseSwap() ");        printList(head);        var st = pairWiseSwap(head);        document.write("<br/>");        document.write("Linked list after calling pairwiseSwap() ");        printList(st);        document.write("");  // This code contributed by aashish1995 </script>
Linked list before calling pairWiseSwap() 1 2 3 4 5 6 7 
Linked list after callingpairWiseSwap() 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) since using constant space to track prev and next
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. 
Implementation:
C++
C
Java
Python3
C#
Javascript
/* 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 <bits/stdc++.h>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* remaining = 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(remaining);      // Return new head of modified list    return newhead;}  /* 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 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 thefield from the nodes.Imagine a case where a node contains many fields, there will be plentyof unnecessary swap calls. */  #include <stdbool.h>#include <stdio.h>#include <stdlib.h>  /* 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 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->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 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 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("");    }}
# Python3 program to pairwise swap# linked list using recursive method  # Linked List Nodeclass Node:          def __init__(self, data):                  self.data = data        self.next = None  # Create and Handle list operationsclass 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 CodelinkedList = 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
// C# program to swap elements// of linked list by changing linksusing 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 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) {            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
<script>// javascript program to swap elements of linked list by changing linksvar head;       class Node {            constructor(val) {                this.data = val;                this.next = null;            }        }      /*     * Function to pairwise swap elements of a linked  It returns head of the     * modified list, so return value of this node must be assigned     */    function pairWiseSwap(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 nodesvar remaining = node.next.next;          // Change headvar 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 */    function printList(node) {        while (node != null) {            document.write(node.data + " ");            node = node.next;        }    }      // Driver program to test above functions                /*         * The constructed linked list is: 1->2->3->4->5->6->7         */        head = new Node(1);        head.next = new Node(2);        head.next.next = new Node(3);        head.next.next.next = new Node(4);        head.next.next.next.next = new Node(5);        head.next.next.next.next.next = new Node(6);        head.next.next.next.next.next.next = new Node(7);          document.write("Linked list before calling pairwiseSwap() ");        printList(head);        head = pairWiseSwap(head);        document.write("<br/>");        document.write("Linked list after calling pairwiseSwap() ");        printList(head);        document.write("");  // This code contributed by umadevi9616</script>
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)
Pairwise swap adjacent nodes of a linked list by changing pointers | Set 2
Rishabh Jindal 2
princiraj1992
Rajput-Ji
rathbhupendra
manoj_n
AmiyaRanjanRout
simranarora5sos
arorakashish0911
aashish1995
umadevi9616
ruhelaa48
adnanirshad158
saurabh1990aror
khushboogoyal499
polymatir3j
hardikkoriintern
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