sbPre这个变量在递归的过程中，是怎么变化的？

import java.sql.*;

public class ArticleTree {

	public static void main(String[] args) {
		new ArticleTree().show();
	}
	
	private void show(){
		Connection conn = null;
		Statement stmt = null;
		ResultSet rs = null;
		
		try {
			Class.forName("com.mysql.jdbc.Driver");
			conn = DriverManager
			.getConnection("jdbc:mysql://localhost/bbs?user=root&password=root");
			stmt = conn.createStatement();
			rs = stmt.executeQuery("select * from article where pid=0");
			while(rs.next()){
				System.out.println(rs.getString("cont"));
				tree(conn,rs.getInt("id"),1);
			}			
		} catch (ClassNotFoundException e) {			
			e.printStackTrace();
		} catch (SQLException e) {			
			e.printStackTrace();
		} finally {
			try {
				if (conn != null){
					conn.close();
					conn = null;
				}
				if (stmt != null){
					stmt.close();
					stmt = null;
				}
				if (rs != null){
					rs.close();
					rs = null;
				}
			} catch (SQLException e) {				
					e.printStackTrace();
			}			
		}
		
	}

	
	private void tree(Connection conn,int id,int level) {
		Statement stmt = null;
		ResultSet rs = null;
		String str = "select * from article where pid =";
		StringBuffer sbPre = new StringBuffer("");
		for(int i=0;i<level; i++){
			sbPre.append("    ");
		}
		try {
			stmt = conn.createStatement();
			rs = stmt.executeQuery(str + id);
			while(rs.next()){
				System.out.println(sbPre + rs.getString("cont"));
				if(rs.getInt("isleaf") != 0){
					tree(conn,rs.getInt("id"),level+1);
				}
			}
		} catch (SQLException e) {
			
			e.printStackTrace();
		} finally {
			try {
				if(rs != null) {
					rs.close();
					rs = null;
				}
				if(stmt != null) {
					stmt.close();
					stmt = null;
				}
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}		
		
		 
		
		
	}

}

下图是article这张表的数据和结构：

下图是程序执行的输出结果：