题目链接：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 head) {
        if(head == null || head.next == null || head.next.next == null) return null;

        // 定义快慢指针 i j
        ListNode i = head, j = head;
        while(i != null && j != null)
        {
            i = i.next;
            j = j.next;
            if(j != null) j = j.next;

            if(i == j)
            {
                i = head;
                while(i != j)
                {
                    i = i.next;
                    j = j.next;
                }
                return i;
            }

        }
        return null;
    }
}