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Pairwise swap elements of a given linked list by changing links
  • Difficulty Level : Medium
  • Last Updated : 03 Dec, 2020

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 <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)
{
    // 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 proceeeding
  
    // 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 head;
}
 
/* Function to add a node at
the begining 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

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 <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 proceeeding
 
    // 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 begining 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;
    }
}
 
/* Druver 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




// 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 proceeeding
 
        // 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




# 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

C#




// C# program to swap elements of
// linked list by changing links
using 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 proceeeding
        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) {
            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

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.

Following is the recursive implementation of the same approach. We change 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 <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* remaing = 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(remaing);
 
    // Return new head of modified list
    return newhead;
}
 
/* Function to add a node at the begining 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;
    }
}
 
/* Druver 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

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 <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* remaing = 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(remaing);
 
    // Return new head of modified list
    return newhead;
}
 
/* Function to add a node at the begining 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;
    }
}
 
/* Druver 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




// 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 remaing = 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(remaing);
 
        // 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




# 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

C#




// C# program to swap elements
// of linked list by changing links
using 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 remaing = 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(remaing);
 
        // 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

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

Pairwise swap adjacent nodes of a linked list by changing pointers | Set 2
This article is contributed by Gautam Kumar. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 

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