问题描述：返回中间节点。链表的（）
题目链接：Leetcode 876. Middle of the Linked List
快慢指针法，没有什么好说的。
代码如下
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution:
def middleNode(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
slow,fast = head,head
while(fast and fast.next):
fast = fast.next.next
slow = slow.next
return slow