public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        boolean hasCircle = false;
        while (slow != null && fast != null) {
            slow = slow.next;
            fast = fast.next;
            if (fast == null) {
                break;
            }
            fast = fast.next;
            if (fast == slow) {
                hasCircle = true;
                break;
            }
        }
        if (!hasCircle) return null;

        ListNode p = head;
        while (p != slow) {
            p = p.next;
            slow = slow.next;
        }
        return p;
    }
}