/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {

        if(head==null || head.next==null){
            return head;
        }
        ListNode f = head;
        ListNode s = head.next;
        ListNode last = head;
        while(f!=null && s!=null){
            //swap nodes
            ListNode temp = s.next;
            s.next = f;
            f.next = temp;
            if(last==head){
                head = s;
            }

            else{
                last.next = s;
            }

            if (temp==null){
                return head;
            }
            else{
                f = temp;
                s = temp.next;
                last = f;
            }
        }
        return head;
    }
}