class Solution {
    public ListNode entryNodeOfLoop(ListNode head) {
        if(head == null || head.next == null){
            return null;
        }
        ListNode quick = head.next.next,slow = head.next;
        boolean isCircle = false;
        while(quick != null && quick.next != null){
            if(quick != slow){
                quick = quick.next.next;
                slow = slow.next;
            }else{
                slow = head;
                while(slow != quick){
                    slow = slow.next;
                    quick = quick.next;
                }
                return slow;
            }
        }
        return null;
    }
}