How to get root category id in admin product grid

Question

I have multi-website, multi-store magento installation. Each store have different root category.

I have created admin users for each store and restricted access to their related store.

In each admin panel product grid I have to display his product only.

So I want to filter each product before display in grid, To do this i need the root category id of current logged in admin.

But whenever I try to get current admin store root category, the value is zero.

So the questions are:

1. How can I get currently logged in admin store's root category?
2. How can I get each product root category?

Thanks in Advance..