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