public static void Drawing(int rowCount)
{
    if(rowCount < 3 || rowCount > 21)
    {
         Console.WriteLine('输入的行数必须位于3与21之间');
         return;
    }

    string printChar = "**************************************";
    string emptyChar = "                                      ";
    StringBuilder printResult = new StringBuilder();
    for(int i=rowCount,j=0;i>0; i--,j++)
    {
         printResult = printResult.Append(printChar.Substring(0,i)).Append(emptyChar.Substring(0,j)).Append("\r\n");
    }
    Console.WriteLine(printResult.ToString());
}
// 手写代码。没测试