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.

Example 1:

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.

Example 2:

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.

我的方法好笨，就是先遍歷一遍得到長度，然後再求中值，再遍歷==

正確的做法：設定兩個快慢指標，慢的每次走一步，快的每次走兩步。當快的指標到達尾部時，慢的指標就是要求的值了，返回即可。但是要判斷快指標是在連結串列尾部還是None。