How To Replace Subcategories With Posts in Sidebar?

I’ve been searching for hours now to solve this. The idea is pretty simple but I can’t find out how to do it.

Categories:

Cat 1
– Subcat 1
– Subcat 2
Cat 2
– Subcat 1
– Subcat 2

Instead of displaying subcategories in the sidebar I want to display the posts of that category. So it would turn out like:

Cat 1
– Post 1
– Post 2
Cat 2
– Post 1
– Post 2

It would be great if you could hide all the posts until you actually click the main category.

Thanks.