Linked List Cycle

 1 /**
 2  * Definition for ListNode.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int val) {
 7  *         this.val = val;
 8  *         this.next = null;
 9  *     }
10  * }
11  */ 
12 public class Solution {
13     /**
14      * @param head: The first node of linked list.
15      * @return: True if it has a cycle, or false
16      */
17     public boolean hasCycle(ListNode head) {  
18         if (head == null || head.next == null) {
19              return false;
20         }
21         ListNode slow = head;
22         ListNode fast = head.next;
23         while (fast != null && fast.next != null) {
24             slow = slow.next;
25             fast = fast.next.next;
26             if (fast == slow) {
27                 return true;
28             }
29         }
30         return false;
31     }
32 }