876. Middle of the Linked List

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.

Note:

• The number of nodes in the given list will be between 1 and 100.

在判断回文链表的时候，分析过如何拿到中间结点了

https://www.cnblogs.com/chucklu/p/10428868.html

直接把结论拿过来用,通过双指针，快指针每次移动2个结点，慢指针每次移动1个结点。

奇数的时候(slow所在的位置是中间位置)
偶数的时候(slow所在的位置，是下半段的第一个结点)

 public ListNode MiddleNode(ListNode head)
        {
            var slow = head;
            var fast = head;
            // Get the middle of the list.
            // Move slow by 1 and fast by 2
            // slow will have the middle mode
            while (fast?.next != null)
            {
                fast = fast.next.next;
                slow = slow.next;
            }

            return slow;
        }