Alternating Widget Styles

I’ve been working on this for a few days now, and I need some help. Here is the basic output idea that I am trying to get from my sidebar widgets:

<div class="sidebar">
<div class="type1">Search stuff here.</div>
<div class="type2">Blog roll here.</div>
<div class="type1">Another widget here.</div>
</div>

Note the alternating css classes. I want to alternate the style of each widget between type1 and type2. I have tried this in functions.php:

'before_widget' => '<div class="type'.$style.'">',

This works to get either 1 or 2 if I define my var in functions.php. How can I create a loop or logic statement that can change the value of $style after each widget is generated?

Thanks!