PHP cycle values to alternate a css class?

Im trying to do what the title says. Use PHP to alternate a css class each time through a loop.

<?php while(have_posts()) : the_post(); ?>
      <div class="post" id="post-<?php the_ID(); ?>">
            <div class="post-content ALTERNATE-CLASS-HERE">
                   ---stuff here---
            </div>
      </div>
<?php endwhile; ?>

I just need to cycle through 2 classes in the div above. Thanks for any help.