trigger UpdateContactOwner on Account (before insert, after insert, before update, after update) {
	set<Id> setAccountIds = new set<Id>() ;
    if(Trigger.isAfter && Trigger.isUpdate){
        for(Account oAccount : Trigger.New){
            if(oAccount.OwnerId != Trigger.oldMap.get(oAccount.Id).OwnerId){
                setAccountIds.add(oAccount.Id) ;
            }
        }
        if(!setAccountIds.isEmpty()){
            list<Account> listAccount = [Select Id, OwnerId, (Select Id, OwnerId from Contacts) from Account Where Id IN: setAccountIds] ;
            if(listAccount != null && !listAccount.isEmpty()){
                list<Contact> listContact = new list<Contact>() ;
                for(Account acc : listAccount){
                    for(Contact con : acc.Contacts){
                        con.OwnerId = acc.OwnerId ;
                        listContact.add(con) ;
                    }
                }
                if(!listContact.isEmpty()) update listContact ;
            }
        }
    }
}