套路：
快慢指针，当两指针相遇时，快指针回到头结点，两个指针继续同步移动一步，直到第二次相遇即为所求。
class Solution: def detectCycle(self, head: ListNode) -> ListNode: if not head or not head.next: return fast = slow = head while fast and fast.next: fast = fast.next.next slow = slow.next if fast == slow: fast = head while fast != slow: fast = fast.next slow = slow.next return slow return
