链表中环的入口结点

 1 　　public ListNode EntryNodeOfLoop(ListNode pHead){
 2         if(pHead==null)
 3             return null;
 4         ListNode slow = pHead, fast = pHead;
 5         while(true){
 6             if(fast==null)
 7                 return null;
 8             slow = slow.next;
 9             fast = fast.next.next;
10             if(slow==fast)
11                 break;
12         }
13         fast = pHead;
14         while(fast!=slow){
15             fast = fast.next;
16             slow = slow.next;
17         }
18         return fast;
19     }