ARTS 打卡第一周 5.25-5.31

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        fast, slow = head, head
        while True:
            if not (fast and fast.next):
                return None
            slow = slow.next
            fast = fast.next.next
            if fast is slow:
                break
        fast = head
        while fast is not slow:
            slow = slow.next
            fast = fast.next
        return fast