LeetCode 876 - 链表的中间结点

class Solution:
    def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]:
        # 头为空直接返回None
        if head == None:
            return head
        slow, fast = (head, head)
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow