linked list cycle ii

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