# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
dummy = ListNode(0)
dummy.next = head
prev = dummy
while prev.next and prev.next.next:
first = prev.next
second = prev.next.next
prev.next, first.next, second.next = second, second.next, first
prev = first
return dummy.next