876. Middle of the Linked List
给你一个linked list，请找出它的middle node，
如长度为偶数，中间有二middle node，则取第二个。
Example 1:
Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.
Input: head = [1,2,3,4,5,6]
Output: [4,5,6]
Explanation: Since the list has two middle nodes with values 3 and 4, we
return the second one.
思路:
linked list无法直接得知长度，所以先设另一linked list为i，使i=head，
将i跑完一遍，便能得知长度。
之后将长度/2，为了使小数点进位，所以加上round()函式。
最后用循环跑出middle node，回传head。
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