链表是否有环
linked-list-cycle
http://www.nowcoder.com/questionTerminal/650474f313294468a4ded3ce0f7898b9
快慢指针
ListNode* fast = head;
ListNode* slow = head;
while(fast != NULL && fast->next != NULL)
{
fast=fast->next->next;
slow=slow->next;
if(slow == fast)
return true;
}
return false;

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