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class Solution:
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    def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
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        if not head or not head.next:
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            return head
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        dummy = ListNode(0)
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        dummy.next = head
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        curr = dummy
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        while curr.next and curr.next.next:
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            first = curr.next
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            second = curr.next.next
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            curr.next = second
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            first.next = second.next
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            second.next = first
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            curr = curr.next.next
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        return dummy.next