如果链表中含有环，如何计算这个环的起点

 

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