# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def middleNode(self, head: Optional[ListNode]) -> Optional[ListNode]:
        # 建立快慢針
        fast = slow = head
        # 快針遍歷，快針結束，慢針過半
        while fast and fast.next:
            # 慢針進1
            slow = slow.next
            # 快針進2
            fast = fast.next.next
        # 回傳過半慢針
        return slow