1class Solution {2    public ListNode swapPairs(ListNode head) {3        ListNode dummy = new ListNode(0, head), prev = dummy;4        while (prev.next != null && prev.next.next != null) {5            ListNode first = prev.next, second = first.next;6            first.next = second.next;7            second.next = first;8            prev.next = second;9            prev = first;10        }11        return dummy.next;12    }13}