LC876 Middle of the Linked List

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    # time O(N) space O(1)
    # main idea: fast slow pointer
    def middleNode(self, head: ListNode) -> ListNode:
        slow = fast = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        return slow