Pairwise Swap Nodes of a given Linked List

15 Jul, 2022
Given a singly linked list, write a function to swap elements pairwise. 
Input : 1->2->3->4->5->6->NULL Output : 2->1->4->3->6->5->NULL
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Input : 1->2->3->4->5->NULL Output : 2->1->4->3->5->NULL
Input : 1->NULL Output : 1->NULL 
For example, if the linked list is 1->2->3->4->5 then the function should change it to 2->1->4->3->5, and if the linked list is then the function should change it to.  
METHOD 1 (Iterative) 
Start from the head node and traverse the list. While traversing swap data of each node with its next node’s data. 
Below is the implementation of the above approach: 
C++
C
Java
Python
C#
Javascript
// C++ program to pairwise swap elements// in a given linked list#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 */void pairWiseSwap(Node* head){    Node* temp = head;      /* Traverse further only if     there are at-least two nodes left */    while (temp != NULL && temp->next != NULL) {        /* Swap data of node with            its next node's data */        swap(temp->data,             temp->next->data);          /* Move temp by 2 for the next pair */        temp = temp->next->next;    }}  /* 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 Codeint main(){    Node* start = NULL;      /* The constructed linked list is:     1->2->3->4->5 */    push(&start, 5);    push(&start, 4);    push(&start, 3);    push(&start, 2);    push(&start, 1);      cout << "Linked list "         << "before calling pairWiseSwap()\n";    printList(start);      pairWiseSwap(start);      cout << "\nLinked list "         << "after calling pairWiseSwap()\n";    printList(start);      return 0;}  // This code is contributed// by rathbhupendra
/* C program to pairwise swap elements in a given linked list */#include <stdio.h>#include <stdlib.h>  /* A linked list node */struct Node {    int data;    struct Node* next;};  /*Function to swap two integers at addresses a and b */void swap(int* a, int* b);  /* Function to pairwise swap elements of a linked list */void pairWiseSwap(struct Node* head){    struct Node* temp = head;      /* Traverse further only if there are at-least two nodes left */    while (temp != NULL && temp->next != NULL) {        /* Swap data of node with its next node's data */        swap(&temp->data, &temp->next->data);          /* Move temp by 2 for the next pair */        temp = temp->next->next;    }}  /* UTILITY FUNCTIONS *//* Function to swap two integers */void swap(int* a, int* b){    int temp;    temp = *a;    *a = *b;    *b = temp;}  /* 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 */    push(&start, 5);    push(&start, 4);    push(&start, 3);    push(&start, 2);    push(&start, 1);      printf("Linked list before calling pairWiseSwap()\n");    printList(start);      pairWiseSwap(start);      printf("\nLinked list after calling pairWiseSwap()\n");    printList(start);      return 0;}
// Java program to pairwise swap elements of a linked listclass LinkedList {    Node head; // head of list      /* Linked list Node*/    class Node {        int data;        Node next;        Node(int d)        {            data = d;            next = null;        }    }      void pairWiseSwap()    {        Node temp = head;          /* Traverse only till there are atleast 2 nodes left */        while (temp != null && temp.next != null) {              /* Swap the data */            int k = temp.data;            temp.data = temp.next.data;            temp.next.data = k;            temp = temp.next.next;        }    }      /* Utility functions */      /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Function to print linked list */    void printList()    {        Node temp = head;        while (temp != null) {            System.out.print(temp.data + " ");            temp = temp.next;        }        System.out.println();    }      /* Driver program to test above functions */    public static void main(String args[])    {        LinkedList llist = new LinkedList();          /* Created Linked List 1->2->3->4->5 */        llist.push(5);        llist.push(4);        llist.push(3);        llist.push(2);        llist.push(1);          System.out.println("Linked List before calling pairWiseSwap() ");        llist.printList();          llist.pairWiseSwap();          System.out.println("Linked List after calling pairWiseSwap() ");        llist.printList();    }}/* This code is contributed by Rajat Mishra */
# Python program to swap the elements of linked list pairwise  # Node class    class Node:      # Constructor to initialize the node object    def __init__(self, data):        self.data = data        self.next = None    class LinkedList:      # Function to initialize head    def __init__(self):        self.head = None      # Function to pairwise swap elements of a linked list    def pairwiseSwap(self):        temp = self.head          # There are no nodes in linked list        if temp is None:            return          # Traverse furthethr only if there are at least two        # left        while(temp and temp.next):              # If both nodes are same,            # no need to swap data            if(temp.data != temp.next.data):                  # Swap data of node with its next node's data                temp.data, temp.next.data = temp.next.data, temp.data              # Move temp by 2 to the next pair            temp = temp.next.next      # Function to insert a new node at the beginning    def push(self, new_data):        new_node = Node(new_data)        new_node.next = self.head        self.head = new_node      # Utility function to print the linked LinkedList    def printList(self):        temp = self.head        while(temp):            print temp.data,            temp = temp.next    # Driver programllist = LinkedList()llist.push(5)llist.push(4)llist.push(3)llist.push(2)llist.push(1)  print "Linked list before calling pairWiseSwap() "llist.printList()  llist.pairwiseSwap()  print "\nLinked list after calling pairWiseSwap()"llist.printList()  # This code is contributed by Nikhil Kumar Singh(nickzuck_007)
// C# program to pairwise swap elements of a linked listusing System;class LinkedList {    Node head; // head of list      /* Linked list Node*/    public class Node {        public int data;        public Node next;        public Node(int d)        {            data = d;            next = null;        }    }      void pairWiseSwap()    {        Node temp = head;          /* Traverse only till there are atleast 2 nodes left */        while (temp != null && temp.next != null) {              /* Swap the data */            int k = temp.data;            temp.data = temp.next.data;            temp.next.data = k;            temp = temp.next.next;        }    }      /* Utility functions */      /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                 Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Function to print linked list */    void printList()    {        Node temp = head;        while (temp != null) {            Console.Write(temp.data + " ");            temp = temp.next;        }        Console.WriteLine();    }      /* Driver program to test above functions */    public static void Main(String[] args)    {        LinkedList llist = new LinkedList();          /* Created Linked List 1->2->3->4->5 */        llist.push(5);        llist.push(4);        llist.push(3);        llist.push(2);        llist.push(1);          Console.WriteLine("Linked List before calling pairWiseSwap() ");        llist.printList();          llist.pairWiseSwap();          Console.WriteLine("Linked List after calling pairWiseSwap() ");        llist.printList();    }}// This code is contributed by Arnab Kundu
<script>  // JavaScript program to pairwise swap // elements of a linked listvar head; // head of list      /* Linked list Node */    class Node {        constructor(val) {            this.data = val;            this.next = null;        }    }    function pairWiseSwap() {var temp = head;          /* Traverse only till there are         atleast 2 nodes left */        while (temp != null && temp.next != null) {              /* Swap the data */            var k = temp.data;            temp.data = temp.next.data;            temp.next.data = k;            temp = temp.next.next;        }    }      /* Utility functions */      /* Inserts a new Node at front of the list. */     function push(new_data) {        /*         * 1 & 2: Allocate the Node & Put in the data         */var new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Function to print linked list */    function printList() {var temp = head;        while (temp != null) {            document.write(temp.data + " ");            temp = temp.next;        }        document.write("<br/>");    }      /* Driver program to test above functions */                  /* Created Linked List 1->2->3->4->5 */        push(5);        push(4);        push(3);        push(2);        push(1);          document.write(        "Linked List before calling pairWiseSwap() <br/>"        );        printList();          pairWiseSwap();          document.write(        "Linked List after calling pairWiseSwap()<br/> "        );        printList();  // This code is contributed by todaysgaurav  </script>
Linked list before calling pairWiseSwap()
1 2 3 4 5 
Linked list after calling pairWiseSwap()
2 1 4 3 5 
Time complexity: O(N)
As we traverse the linked list only once.
Auxiliary Space: O(1)
As constant extra space is used.
METHOD 2 (Recursive) If there are 2 or more than 2 nodes in Linked List then swap the first two nodes and recursively call for the rest of the list.
Below image is a dry run of the above approach: 
Below is the implementation of the above approach:
C++
C
Java
Python3
C#
Javascript
/* Recursive function to pairwise swap elements   of a linked list */void pairWiseSwap(node* head){    /* There must be at-least two nodes in the list */    if (head != NULL && head->next != NULL) {          /* Swap the node's data with data of next node */        swap(head->data, head->next->data);          /* Call pairWiseSwap() for rest of the list */        pairWiseSwap(head->next->next);    }}  // The code is contributed by Gautam goel (gautamgoel962)
/* Recursive function to pairwise swap elements   of a linked list */void pairWiseSwap(struct node* head){    /* There must be at-least two nodes in the list */    if (head != NULL && head->next != NULL) {          /* Swap the node's data with data of next node */        swap(head->data, head->next->data);          /* Call pairWiseSwap() for rest of the list */        pairWiseSwap(head->next->next);    }}
/* Recursive function to pairwise swap elements   of a linked list */static void pairWiseSwap(node head){    /* There must be at-least two nodes in the list */    if (head != null && head.next != null) {          /* Swap the node's data with data of next node */        swap(head.data, head.next.data);          /* Call pairWiseSwap() for rest of the list */        pairWiseSwap(head.next.next);    }}  // This code contributed by aashish1995 
# Recursive function to pairwise swap elements of a linked list def pairWiseSwap(head):      # There must be at-least two nodes in the list     if (head != None and head.next != None):           # Swap the node's data with data of next node         swap(head.data, head.next.data);           # Call pairWiseSwap() for rest of the list         pairWiseSwap(head.next.next);  # This code is contributed by _saurabh_jaiswal
/* Recursive function to pairwise swap elements   of a linked list */static void pairWiseSwap(node head){    /* There must be at-least two nodes in the list */    if (head != null && head.next != null) {          /* Swap the node's data with data of next node */        swap(head.data, head.next.data);          /* Call pairWiseSwap() for rest of the list */        pairWiseSwap(head.next.next);    }}    // This code contributed by aashish1995
<script>/* Recursive function to pairwise swap elements   of a linked list */  function pairWiseSwap(head){    /* There must be at-least two nodes in the list */    if (head != null && head.next != null) {           /* Swap the node's data with data of next node */        swap(head.data, head.next.data);           /* Call pairWiseSwap() for rest of the list */        pairWiseSwap(head.next.next);    }}        // This code is contributed by avanitrachhadiya2155  </script>
Time complexity: O(n)
Auxiliary Space: O(1)
As it is a tail recursive function, function call stack would not be build and thus no extra space will be used.
The solution provided here swaps data of nodes. If the data contains many fields (for example a linked list of Student Objects), the swap operation will be costly. See the below article for a better solution that works well for all kind of linked lists
Pairwise Swap Nodes by Changing Links
Please write comments if you find any bug in the above code/algorithm, or find other ways to solve the same problem.
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