给定一个带有头结点 head 的非空单链表，返回链表的中间结点。如果有两个中间结点，则返回第二个中间结点

class Node {
    public int data;
    public Node next;  //存储对象引用

    public Node(int data) {
        this.data = data;  //没有初始化next的引用,不知道next当前指向哪个节点
    }
}

public class SingleList {
    public Node head;//定位头节点的引用

    public void addFirst(int data) {
        Node node = new Node(data); //构造要插入的对象
        if(this.head == null) {
            this.head = node;
            return;
        } else {
            node.next = this.head;
            this.head = node;
        }
    }

    public void show(){
        Node cur=this.head;   //从头开始
        while(cur!=null){
            System.out.print(cur.data+" ");
            cur=cur.next;
        }
        System.out.println();
    }

    public Node middle(){
        Node fast=this.head;
        Node slow=this.head;
        while(fast!=null&&fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }
        return slow;
    }
}

```java
public class TestDemo {
    public static void main(String[] args) {
SingleList singleList=new SingleList();
        singleList.addFirst(2);
            singleList.addFirst(1);
            singleList.addFirst(3);
            singleList.addFirst(4);
            singleList.addFirst(2);
        singleList.show();

Node middle=singleList.middle();
        System.out.println(middle.data);

    }
}