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 p1 = pHead;
        ListNode p2 = pHead;

        while(p1 != null && p2 != null){
            p1 = p1.next;
            if(p2.next != null){
                p2 = p2.next.next;    
            }
            //相遇说明存在环
            if(p1 == p2){
                //cur节点从头出发，Cur走一步，p1走一步，两者相遇处即为环的入口
                ListNode cur = pHead;
                while(cur != p1){
                    cur = cur.next;
                    p1 = p1.next;
                }
                return cur;
            }
        }
        //不存在环， 返回null
        return null;
    }
}