双指针， 设置慢指针为 slow， 快指针为 fast, fast 指针每次移动两位, slow 每次移动一位;则 fast 到达末尾时, slow 为中间节点

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @return ListNode类
#
class Solution:
    def middleNode(self , head: ListNode) -> ListNode:
        # write code here
        if not head or not head.next: return head
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow