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