Linked List Cycle Start Point

public ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }

        ListNode fast = head, slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;

            if (fast == slow) {
                while (head != slow) {
                    head = head.next;
                    slow = slow.next;
                }
                return head;
            }
        }

        return null;
    }