Linked List Cycle

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

Follow up:

Can you solve it without using extra space?

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12         
13         ListNode *fastPoint = head;
14         ListNode *slowPoint = head;
15         
16         if(fastPoint == NULL || fastPoint->next == NULL)
17             return false;
18             
19         while(fastPoint->next != NULL && fastPoint->next->next != NULL)
20         {
21             fastPoint = fastPoint->next->next;
22             slowPoint = slowPoint->next;
23             if(fastPoint == slowPoint)
24                 return true;
25         }
26         
27         return false;
28     }
29 };