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主要思路：用两个指针，步长分别为1和2，遍历，相遇时即表示有环。

bool hasCircle(Node* head, Node* &encounter){         Node *fast = head, *low = head;         while(fast && fast->next)        {               fast = fast->next->next;              low = low->next;              if(fast == low)               {                   encounter = fast;                   return true;               }          }         // fast == NULL || fast->next == NULL         encounter = NULL;         return false;}

Node* findEntry(Node* head, Node* encounter){           Node *p1 = head, *p2 = encounter;         while(p1 != p2)         {                p1 = p1->next;               p2 = p2->next;          }         return p1;}

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