Pairwise swap elements of a given linked list by changing links

2.6

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

/* 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<stdio.h>
#include<stdlib.h>
#include<stdbool.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 null;
        }

        // 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

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

/* 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<stdio.h>
#include<stdlib.h>
#include<stdbool.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("");

    }
}

 Linked list before calling  pairWiseSwap() 1 2 3 4 5 6 7
 Linked list after calling  pairWiseSwap() 2 1 4 3 6 5 7 

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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