876. Middle of the Linked List
給你一個linked list，請找出它的middle node，
如長度為偶數，中間有二middle node，則取第二個。
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無法直接得知長度，所以先設另一linked list為i，使i=head，
將i跑完一遍，便能得知長度。
之後將長度/2，為了使小數點進位，所以加上round()函式。
最後用迴圈跑出middle node，回傳head。
C Code