876. Middle of the Linked List

https://leetcode.com/problems/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.

找链表中点，当长度为偶数时返回中点中的第二个。快慢指针一起走，快结点每次走两格，当快结点不能再走时慢结点走到链表一半，也就是中点位置。如果要返回偶数链表第一个中点，初始时让快指针提前一个身位。

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def middleNode(self, head: ListNode) -> ListNode:
        s = f = head
        while f and f.next:
            s = s.next
            f = f.next.next
        return s