中序遍历的操作如下：
1）中序遍历左子树；
2）访问根节点；
3）中序遍历右子树；
public void inOrder() {
        Node current = root;
        //把LinkedList作为栈使用
        LinkedList<Node> s = new LinkedList<Node>();
        while (current != null || !s.isEmpty()) {
            while (current != null) {
                s.addFirst(current);
                current = current.left;
            }
            if (!s.isEmpty()) {
                current = s.removeFirst();
                System.out.print(current.data + " -> ");
                current = current.right;
            }
        }
    }