Algorithm

We can use a recursive function to swap nodes in pair. If the list is empty or has only one node, return the node. Else, Swap the first two nodes and recursively call the function for the rest of the list starting from the third node. Link the swapped pair with the result of the recursive call at end.

Code

#include <bits/stdc++.h>
using namespace std;

// Definition of singly linked list node
struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
    // Function to swap nodes pairwise recursively
    ListNode* swapPairs(ListNode* head) {
        // Base case: If list is empty or has only one node
        if (!head || !head->next)
            return head;

        // Store the second node
        ListNode* second = head->next;

        // Recursively swap the rest of the list
        head->next = swapPairs(second->next);

        // Point second node to first
        second->next = head;

        // Return new head after swap
        return second;
    }
};

// Function to print linked list
void printList(ListNode* head) {
    while (head) {
        cout << head->val << " ";
        head = head->next;
    }
}

int main() {
    // Creating linked list 1->2->3->4
    ListNode* head = new ListNode(1);
    head->next = new ListNode(2);
    head->next