Linked List Cycle

使用快慢指针，如果两个指针在遍历的过程中相遇，则链表有环，否则无环。

class Solution {public:    bool hasCycle(ListNode *head) {        ListNode *fast = head, *slow = head;                while(fast && fast->next)        {            slow = slow->next;            fast = fast->next->next;            if(slow == fast)                return true;        }        return false;    }};