/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        if(pHead==null||pHead.next==null||pHead.next.next==null)
            return null;
        
        ListNode fast=pHead.next.next;
        ListNode slow=pHead.next;
        
        /先判断有没有环
        
        while(fast!=slow){
            if(fast.next!=null&&fast.next.next!=null){
                fast=fast.next.next;
                slow=slow.next;
            }else{
                没有环，返回
                return null;
            }
        }
        
        循环出来就是有环，且此时fast==slow
        
        fast=pHead;
        while(fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }
}