<p>There are <code>n</code> items each belonging to zero or one of <code>m</code> groups where <code>group[i]</code> is the group that the <code>i</code>-th item belongs to and it's equal to <code>-1</code> if the <code>i</code>-th item belongs to no group. The items and the groups are zero indexed. A group can have no item belonging to it.</p>
<p>Return a sorted list of the items such that:</p>
<ul>
<li>The items that belong to the same group are next to each other in the sorted list.</li>
<li>There are some relations between these items where <code>beforeItems[i]</code> is a list containing all the items that should come before the <code>i</code>-th item in the sorted array (to the left of the <code>i</code>-th item).</li>
</ul>
<p>Return any solution if there is more than one solution and return an <strong>empty list</strong> if there is no solution.</p>
