Pairwise swap elements of a given linked list by changing links?

Question

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

Answer 1

Its simple you need to treat two nodes as one node and traverse.

Assuming the following is your node of the linklist

struct node
{
    int data;
    struct node *next;
};

Code to swap pairwise node

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;
    }
}