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.

想法：通過兩個指針，一個稱為快指針，一個稱為慢指針，慢指針更新時指向它的下一個節點，而快指針更新時指向它的下下個節點，當快指針到達鏈表尾端時，慢指針剛好指向鏈表中間的位置。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* middleNode(struct ListNode* head) {
    struct ListNode* fast = head;
    struct ListNode* slow = head;
    while(fast && fast->next){
        fast = fast->next->next;
        slow 
 
= slow ->next;
    }
    return slow;
}

leetcode-876 Middle of the Linked List