trigger Example123 on Case (before insert, before update) {

    public List <Account> lstAcc =  new List<Account>();
    public Id accountOwner;
   /* For (Case c : Trigger.new){
        lst.add(c.id);
    }*/
    //system.debug('******' +lst);
     list<case> cc= [select id,AccountId, case.Account.OwnerId, AdditionalCon__c from case where id =: Trigger.new];
     system.debug('******' +cc);

      for(Case cas : cc){
      lstAcc = [Select Id, Ownerid from Account where Id  =: cas.AccountId];
      }
     for (Case c: cc){
       // system.debug('c.account.Ownerid**********'+c.account.Ownerid);
         if((Trigger.isInsert || Trigger.isUpdate ) && Trigger.isBefore ){  //Before Insert
            //system.debug('c.account.Ownerid**********'+c.account.Ownerid);
             if(c.AdditionalCon__c ==null ){

                for(Account a : lstAcc ){
                    if(c.AccountId == a.Id){
                    accountOwner = a.OwnerId;
                    }
                }

               system.debug('AccountOwner**********'+accountOwner);
               c.AdditionalCon__c = accountOwner;
               system.debug('AdditionalCon :' +c.AdditionalCon__c);
           }
       }
          //lst.add(c);
    }

        //update lst;
}