双指针模板

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https://www.cnblogs.com/bonelee/p/11789330.html   5个模板       """     def hasCycle(self, head):         # write your code here         slow, fast = head, head         while fast and fast.next and fast.next.next:             slow = slow.next             fast = fast.next.next             if slow == fast:                return True         return False