Linked List Cycle - LeetCode

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

思路：维护两个指针，一快一慢，判断两个指针能否相遇。

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