C Program For Pairwise Swapping 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 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 <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;

    // Change head before proceeding
    *head = curr;

    // Traverse the list
    while (true)
    {
        struct 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;
    }
}

/* 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 of 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 code
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("
    Linked list before calling  pairWiseSwap() ");
    printList(start);

    pairWiseSwap(&start);

    printf("
    Linked list after calling  pairWiseSwap() ");
    printList(start);

    getchar();
    return 0;
}

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.

Auxiliary Space: O(1)

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. 

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* 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 of 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 code
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("
    Linked list before calling  pairWiseSwap() ");
    printList(start);

    // NOTE THIS CHANGE
    start = pairWiseSwap(start);

    printf("
    Linked list after calling  pairWiseSwap() ");
    printList(start);

    return 0;
}

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: O(n)

Auxiliary Space: O(n)

Please refer complete article on Pairwise swap elements of a given linked list by changing links for more details!