【leetcode】Linked List Cycle

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