题解 | #链表中环的入口结点#

import java.util.*;
/*
 public class ListNode {
    int val;
    ListNode next = null;

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

    public ListNode EntryNodeOfLoop(ListNode pHead) {
        ListNode slow = yesNoStrike(pHead);
       //如果无链表则返回为空
        if(slow == null){
            return null;
        }
        //快指针返回到头结点
        ListNode fast = pHead;
        //再次相遇即是环入口
        while(fast != slow){
            fast = fast.next;
            slow = slow.next;
        }
        return slow;
    }
     public ListNode yesNoStrike(ListNode pHead) {
        //如果无链表则返回为空
        if(pHead == null){
            return null;
        }
        //快慢指针
        ListNode fast = pHead;
        ListNode slow = pHead;
        //如果没环快指针会先到链表尾
        while(fast != null && fast.next != null){
            //快指针移动两步
            fast = fast.next.next;
            //慢指针移动一步
            slow = slow.next;
            //快慢指针碰撞则相遇
            if(fast == slow){
                return slow;
            }
        }
        return null;  
    }
}