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) {
        if(pHead==null){
            return null;
        }
        ListNode slow=check(pHead);
        if(slow==null){
            return null;
        }
        ListNode fast=pHead;
        while(slow!=fast){
            slow=slow.next;
            fast=fast.next;
        }
        return slow;
    }
    private static ListNode check(ListNode pHead){
        ListNode fast=pHead;
        ListNode slow=pHead;
        while(fast.next!=null&&fast.next.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow){
                return slow;
            }
        }
        return null;
    }
}