The question is, was that grid valid at all times or did you use trial and error?

A very, very easy way to generate a valid sudoku (filled grid) is:
<pre>
a b c d e f g h i
g h i a b c d e f
d e f g h i a b c
i a b c d e f g h
f g h i a b c d e
c d e f g h i a b
h i a b c d e f g
e f g h i a b c d
b c d e f g h i a
</pre>