Linked List Cycle(Medium)

Given a linked list, determine if it has a cycle in it.

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

fast and slow pointer

public boolean hasCycle(ListNode head) {
        if(head==null || head.next==null) return false;
        ListNode fast=head,slow=head;
        while(fast!=null&& fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow) return true;
        }
        return false;
 }