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 public class Solution {
 2     public boolean hasCycle(ListNode head) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         ListNode fast = head, slow = head;
 6         while(fast != null && fast.next != null){
 7             fast = fast.next.next;
 8             slow = slow.next;
 9             if(fast == slow) return true;
10         }
11         return false;
12     }
13 }