找出链表环的入口

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

        ListNode(int val) {
            this.val = val;
        }
    }
    public ListNode EntryNodeOfLoop2(ListNode pHead){
        //参考的别人的代码：快慢指针+公式推导
        /*
        推导结果:如果存在循环
        (1)第一次出发：快指针（一次走两步），慢指针一次一步，最终一定会在环的某点相遇
        (2)第二次出发：两个指针分别从头结点和相遇节点出发，最后一定会在环入口节点相遇
         */
        if(pHead==null){
            return null;
       }else{
            ListNode low= pHead;
            ListNode fast = pHead;
           while(fast!=null &&fast.next!=null){
                fast = fast.next.next;
                low = low.next;
                if(fast==low) break;
           }
           //没找到环
           if(fast==null || fast.next==null) return null;

           //找到环，根据结论2寻找环入口
           fast=pHead;
           while(fast!=low){
               fast = fast.next;
               low = low.next;
           }
           return fast;
       }
    }

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

        ListNode(int val) {
            this.val = val;
        }
    }
public ListNode EntryNodeOfLoop(ListNode pHead)
    {
            ListNode fast=pHead;
            ListNode low=pHead;
            while(fast!=null&&fast.next!=null){
                fast=fast.next.next;
                low=low.next;
                if(fast==low)
                    break;
            }
            if(fast==null||fast.next==null)
                return null;
            low=pHead;
            while(fast!=low){
                fast=fast.next;
            low=low.next;
        }
        return low;
    }