LeetCode 141. Linked List Cycle

 1 /**
 2  * Definition for singly-linked list.
 3  */
 4 class ListNode {
 5     int val;
 6     ListNode next;
 7 
 8     ListNode(int x) {
 9         val = x;
10         next = null;
11     }
12 }

 1 　　public boolean hasCycleBySet(ListNode head) {
 2         Set<ListNode> set = new HashSet<>();
 3         while(head != null){
 4             if(set.contains(head))
 5                 return true;
 6             else
 7                 set.add(head);
 8             head = head.next;
 9         }
10         return false;
11     }

 1 　　public boolean hasCycle(ListNode head) {
 2         if (head == null || head.next == null) {
 3             return false;
 4         }
 5         ListNode slow = head;
 6         ListNode fast = head.next;
 7         while (slow != fast) {
 8             if (fast == null || fast.next == null) {
 9                 return false;
10             }
11             slow = slow.next;
12             fast = fast.next.next;
13         }
14         return true;
15     }