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class Solution {
    public ListNode swapPairs(ListNode head) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;

        ListNode prev = dummy;
        while (prev.next != null && prev.next.next != null) {
            ListNode a = prev.next;
            ListNode b = a.next;

            prev.next = b;
            a.next = b.next;
            b.next = a;

            prev = a;
        }
        return dummy.next;
    }
}