24. Swap Nodes in Pairs.py

# 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: ListNode) -> ListNode:
        if not head or not head.next:
            return head
        dummy = ListNode(-1)
        dummy.next = head
        prev = dummy
        while prev.next and prev.next.next:
            first, second = prev.next, prev.next.next
            next = second.next
            prev.next = second
            second.next = first
            first.next = next
            prev = first
        return dummy.next