leetcode LinkedListCycle

import utils.ListNode;

public class LinkedListCycle {
    /*
        Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
        Note: Do not modify the linked list.
     */
    public ListNode detectCycle(ListNode head) {
        if (head == null)
            return null;
        ListNode slow = head;
        ListNode fast = head;
        while(true) {
            if (fast != null && fast.next!= null ){
                fast = fast.next.next;
                slow = slow.next;
            }
            else {
                return null;
            }
            if (fast == slow){
                while(head != slow){
                    head = head.next;
                    slow = slow.next;
                }
                return slow;
            }
        }

    }
}