/**

 * Definition for singly-linked list.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) { val = x; }

 * }

 */

class Solution {

    public ListNode swapPairs(ListNode head) {

        ListNode dummy = new ListNode(0);

        dummy.next = head;

        ListNode p = dummy;

        while(p.next != null && p.next.next != null) {

            ListNode s1 = p.next;

            ListNode s2 = p.next.next;

            // step 1: p -> s2

            p.next = s2;

            // step 2: s1 -> s2.next

            s1.next = s2.next;

            // step 3: s2 -> s1

            s2.next = s1;

            // step 4: p = s1

            p = s1;

        }

        return dummy.next;

    }

}