Pairwise swap 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 a C implementation that changes links instead of swapping data.

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

/* 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;
    }
}
  
/* 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 rathbhupendra

chevron_right


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
#include
#include

/* 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

filter_none

edit
close

play_arrow

link
brightness_4
code

// 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

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// 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

chevron_right


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: 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 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++

filter_none

edit
close

play_arrow

link
brightness_4
code

/* 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

chevron_right


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
#include
#include

/* 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

filter_none

edit
close

play_arrow

link
brightness_4
code

// 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("");
    }
}

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// 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

chevron_right



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



My Personal Notes arrow_drop_up



Article Tags :
Practice Tags :


10


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.