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 }