1. 題目描述
English:
Given a non-empty, singly linked list with head node head, return a middle node of linked list.
If there are two middle nodes, return the second middle node.
中文：
給定一個帶有頭節點的非空單鏈表head，返回鏈表的中間節點。
如果有兩個中間節點，則返回第二個中間節點。
Example:
Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3. (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.
Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.
2. 解題思路（雙指針）
快慢指針，就是兩個指針，慢指針一次走一步，快指針一次走兩步，那麼這裏當快指針走到末尾的時候，慢指針剛好走到中間，這樣就在一次遍歷中，且不需要額外空間的情況下解決了問題
3. python 代碼
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def middleNode(self, head: ListNode) -> ListNode:
slow = fast = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
return slow