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;