141. Linked List Cycle

 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         while(fast != null && fast.next != null){
16             slow = slow.next;
17             fast = fast.next.next;
18             if(slow == fast)
19                 return true;
20         }
21         return false;
22     }
23 }