leetcode中如何获取链表的中间结点

# Definition for singly-linked list.# class ListNode:#     def __init__(self, x):#         self.val = x#         self.next = None
class Solution:    def middleNode(self, head: ListNode) -> ListNode:        if head is None or head.next is None:            return head        # 辅助列表        '''        l = []        while head:            l.append(head)            head = head.next        return l[len(l)//2]        '''        # 快慢指针        # slow 走一步， fast每次走l两步，当fast走到末尾时，slow走到中间        slow = head        fast = head        while fast and fast.next:            slow = slow.next            fast = fast.next.next         return slow