LeetCode-141. Linked List Cycle

思路:双指针法，设置快慢双指针。假如有环，快指针一定会和慢指针相遇。假如无环，快指针一定首先到达链表尾部。

    bool hasCycle(ListNode *head) {        ListNode *slow = head;        ListNode *fast = head;        while(fast !=NULL){            slow = slow->next;            if(fast->next)                fast = fast->next->next;            else                return false;                        if(slow==fast)                return true;        }        return false;    }