NC3：链表中环的入口节点

/**
* Definition for singly-linked list.
* class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
   public ListNode detectCycle(ListNode head) {
       if(head==null || head.next==null){
           return null;
       }
       ListNode fast=head;
       ListNode slow=head;
       while(fast!=null && fast.next!=null){
           fast=fast.next.next;
           slow=slow.next;
           if(slow==fast){//利用快慢指针找相遇点
               fast=head;
               while(fast!=slow){
                   slow=slow.next;//设置以相同速度的新指针从起始位置出发
                   fast=fast.next;
               }
               return slow;
           }
       }
       return null;
   }
}