public class Solution {
    public ListNode detectCycle(ListNode head) {

        ListNode fast = head;
        ListNode slow = head;

        while (fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow){
                ListNode cur = head;
                while (cur != slow){
                    cur  = cur.next;
                    slow = slow.next;
                }
                return cur;
            }
        }
        return null;
    }
}