141.linked list cycle

 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class  {
    public boolean hasCycle(ListNode head) {
        HashSet<ListNode> hashset = new HashSet<>();
        while(head!=null){
            if(hashset.contains(head))
                return true;
            else{
                hashset.add(head);
                head = head.next;
            }
        }
        return false;
    }
}

 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class  {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        
        while(fast!=null&&fast.next!=null){  // in case: [1] only one element
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow)
                return true;
        }
        return false;
    }
}