trigger LeadAssignmentTrigger on Lead (before insert, before update) {
    List<String> postcodenames = new List<String>();
//    List<Lead> leadsToUpdate = new List<Lead>();  // not required
//build a list of postcode value that we will search for
    for (Lead lead : Trigger.new) {    
        if (lead.PostalCode != NULL) {
          // Find the sales rep for the current zip code
          postcodenames.add(lead.PostalCode);
        }
     }

//run the SOQL outside the FOR loop using the list we built of postcode values
     List< Territory_Zip_Codes__c> zipList = [select id, Name from Territory_Zip_Codes__c 
                             where Name IN :postcodenames];     
         
    //now we need a map indexed by postcode name
    Map<String,Id> mapZipByName = new Map<String,Id>();
    for(Territory_Zip_Codes__c zip : zipList) {
        mapZipByName.put(zip.name,zip.Id);
    }

//now all the background work is done, do the actual processing of the records in the trigger
    for (Lead lead : Trigger.new) {
        // if you found one
        if (mapZipByName.containsKey(lead.PostalCode ) ) {   
            //assign the lead owner to the zip code owner
            lead.Territory_Zip_Code__c = mapZipByName.get(lead.PostalCode).Id;
         
//              leadsToUpdate.add(lead);
//no need to add them to an "update" list as we are in a before trigger. and it will take care of itself.
         
        }
    }
}