https://leetcode.com/problems/linked-list-cycle/description/

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

time: o(n) space: o(1)

Follow up:
Can you solve it without using extra space?

if you use extra space, then it means using hashMap<val, listNode> time: o(n) space: o(n)

LC_141. Linked List Cycle