判断链表是否包含环

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func hasCycle(head *ListNode) bool {     if head==nil||head.Next==nil{         return false     }     p1,p2:=head,head.Next     for p2!=nil{         p1=p1.Next         p2=p2.Next         if p2!=nil{             p2=p2.Next         }         if p1==p2{             return true         }     }     return false }