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 || pHead.next==null){
            return null;
        }
        if(pHead==pHead.next){
            return pHead;
        }
        ListNode p=pHead,q=pHead;
        while(q.next!=null && q.next.next!=null){
            p=p.next;
            q=q.next.next;
            if(p==q){
                q=pHead;
                while(p!=q){
                    p=p.next;
                    q=q.next;
                }
                return p;
            }
        }
        return null;
    }
}