点滴积累【C#】---1、1、2、3、5、8、13、21....计算第30位(递归)！

protected void Button1_Click(object sender, EventArgs e)
{
if (tb1.Text != "" && tb1.Text != null)
{
if (!Isnum(tb1.Text))
{
Response.Write("<script type='text/javascript'>alert('请输入数字');</script>");
}
else
{
int a = Convert.ToInt32(tb1.Text);
tb2.Text = Convert.ToString(Foo(a));
}
}
else
{
Response.Write("<script type='text/javascript'>alert('不能为空');</script>");
}
}
public static int Foo(int i)
{
if (i < 0)
{
return 0;
}
else if (i > 0 && i <= 2)
{
return 1;
}
else
{
return Foo(i - 1) + Foo(i - 2);
}
}
public static bool Isnum(string s)
{
string pattern = @"^\d*$";
return Regex.IsMatch(s, pattern);
}