java如何确定一个链表有环及入口节点

//定义一个链表

   class   List{

     public   int var;

     public   List next;

//有参构造

     public List( int var) {

         this .var = var;

     }

//无参构造

     public List() {

     }

     //创建一个带环的链表

     public   List Create(){

         List a = new List( 1 );

         List b = new List( 2 );

         List c = new List( 3 );

         List d = new List( 4 );

         List e = new List( 5 );

         List f = new List( 6 );

         a.next = b;

         b.next =c;

         c.next = d;

         d.next =e;

         e.next = f;

         f.next =d;

         return   a;

     }

//判断是否有环，并找到相遇的节点

public   List MeetingNode(List node){

     List slow = new List();

     List fast = new List();

     if (node== null ) return   null ;

     slow = node.next;

     if (slow== null ) return   null ;

     fast=slow.next;

     while (fast!= null && slow!= null ){

         if (fast==slow){

             return fast; //fast追上了slow，确定是一个有环的链表；

         }

         slow = slow.next;

         fast = fast.next;

         if (fast!= null ){

             fast = fast.next;

         }

     }

    return null ;

}

   public   List Enterdear(List node){

         if (node== null ) return null ;

         if (MeetingNode(node)== null ) return null ;

         int count = 1 ;

         List res2;

         List res1 = MeetingNode(node);

         while (res1.next!=MeetingNode(node)){

             res1 = res1.next;

             count++;

         }

         res1 = node;

         for ( int i = 0 ;i<count;i++){

             res1 =res1.next;

         }

         res2 = node;

         while (res1!=res2 && res1!= null && res2!= null ){

             res1 = res1.next;

             res2 = res2.next;

         }

         return res1;

     }

}

ublic class Deom {

     public static void main(String[] args) {

        List SB = new List();

        List res = SB.Create();

        List dear= SB.Enterdear(res);

        System.out.println(dear.var);

     }

}