Given the head
of a singly linked list, return the middle node of the linked list.
If there are two middle nodes, return the second middle node.
Example 1:
1 > 2 > (3) > 4 > 5
Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.
Example 2:
1 > 2 > 3 > (4) > 5 > 6
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.
Constraints:
 The number of nodes in the list is in the range
[1, 100]
.  1 <= Node.val <= 100

