采用递归的方法获得一棵树的所有叶节点

暴汗一个:我突然忘记了怎么写递归....
由于工作原因需要统计某一个文件夹下最底层的文件夹的名字, 立马想到上学时候学的统计叶节点的"故事"(...想起了数据结构上那个风姿绰约昏倒一大片的两脚圆规...永远的痛啊),于是敲定了用递归.
此处借用第一行...
以下是补上的功课..

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Collections;

namespace CasesExport
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        /**//// <summary>
        /// To open the "FolderBrowserDialog" and get the selected path
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnFolder_Click(object sender, EventArgs e)
        {
            FolderBrowserDialog FBDialog = new FolderBrowserDialog();
            FBDialog.ShowNewFolderButton = true;
            FBDialog.ShowDialog();
            tbFolder.Text = FBDialog.SelectedPath;
        }
        /**//// <summary>
        /// To open the "OpenFileDialog" and set the txt file to record the folder names
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnFile_Click(object sender, EventArgs e)
        {
            OpenFileDialog OFDialog = new OpenFileDialog();
            OFDialog.ShowDialog();
            tbFile.Text = OFDialog.FileName;
        }
        /**//// <summary>
        /// Mainly work:
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnOK_Click(object sender, EventArgs e)
        {
            if (this.tbFolder.Text.Length == 0 || this.tbFile.Text.Length == 0)
            {
                return;
            }
            try
            {
                DirectoryInfo DInfo = new DirectoryInfo(this.tbFolder.Text.Trim());
                ArrayList al = new ArrayList();
                GetLeafNodeFolder(DInfo, ref al);
                string StrContent = null;
                foreach (string folderName in al)
                {
                    StrContent += folderName + "\r\n";
                }
                File.WriteAllText(this.tbFile.Text, StrContent);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
            finally
            {
                this.Dispose();
            }
            
        }
        /**//// <summary>
        /// Use recursion to get the leaf node and post them to an ArrayList
        /// </summary>
        /// <param name="dri"></param>
        /// <param name="al"></param>
        private void GetLeafNodeFolder(DirectoryInfo dri, ref ArrayList al)
        {
            if (dri.GetDirectories().Length == 0)
            {
                al.Add(dri.Name);
            }
            else
            {
                foreach (DirectoryInfo subDri in dri.GetDirectories())
                {
                    GetLeafNodeFolder(subDri, ref al);
                }
            }
        }

        private void btnCancle_Click(object sender, EventArgs e)
        {
            this.Dispose();
        }
    }
}

试图用循环的方法来解决这个问题,但是未能理出很好的头绪,主要是无法确定一个未知文件夹到底有多"深", 看来在这些最基本的问题上还是需要加强的.