[SalesForce] How to write a rollup summary trigger for a lookup relationship

I changed my trigger like this to make it work for both insert and delete.

trigger rollupTrigger on Employee__c (after insert, after update, after delete) 
{
    Set<Id> orgIds = new Set<Id>();
    List< Organization__c> updates = new List<Organization__c>();
    Employee__c[] emp = null;

    if(Trigger.isInsert||Trigger.isUpdate)
    {
        emp = Trigger.new;
    }
    else if(Trigger.isDelete)
    {
        emp = Trigger.old;
    }

    for (Employee__c e : emp) 
    {
        if(e.Organization__c != null)
            orgIds.add(e.Organization__c);
    }

    for (AggregateResult ar : [select Organization__c i, sum(Salary__c) su, 
        min(Salary__c) mi, max(Salary__c) ma,count(id) ids from employee__c 
        where Organization__c in :orgIds group by Organization__c])
    {
        updates.add(new Organization__c(Id = (Id)ar.get('i'),
            totalsalary__c = (Decimal)ar.get('su'),min_salary__c = (Decimal)ar.get('mi'),
            max_salary__c = (Decimal)ar.get('ma'),total_emp_in_org__c = (Decimal)ar.get('ids')));
    }

    update updates;
}

The About Roll-Up Summary Fields documentation says:

Long text area, multi-select picklist, Description, system fields like Last Activity, cross-object formula fields, and lookup fields (such as the Product Code field) cannot be used in the field column of roll-up summary filters.