Leetcode 24 两两交换链表中的节点

给你一个链表，两两交换其中相邻的节点，并返回交换后链表的头节点。你必须在不修改节点内部的值的情况下完成本题（即，只能进行节点交换）。

用哨兵节点dummy，然后每次交换两个节点，然后移动到下一组节点。

# 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]:

        if not head or not head.next:
            return head

        dummy = ListNode(0)
        dummy.next = head
        current = dummy

        while current.next and current.next.next:

            first = current.next
            second = current.next.next

            current.next = second
            first.next = second.next
            second.next = first

            current = first

        return dummy.next