141. Linked List Cycle

//Time: O(n), Space: O(1)   
 public boolean hasCycle(ListNode head) {
        if (head == null) {
            return false;
        }
        
        ListNode slow = head;
        ListNode fast = head;
        
        while (fast != null  && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            
            if (slow == fast) {
                return true;
            }
        }
        
        return false;
    }