Can anyone review this code, and help me see if there are any glaring vulnerabilities for failure, or room for improvement? I'm very new to coding (do my ugly variable names make it obvious? :P ), so even though this works, and I've tested it via manual entry, manual changing, and mass uploading via the data loader, I'm still not 100% ready to push towards production.. 

Key (Since some things changed from original post)

Lead Object

left5zip__c = custom field, basically if len(PostalCode >4) then it's the left 5 digits

mikezip__c = custom lookup field to the ZipCodeTable__c object

ZipCode Object

ZipCodeID__c = 5 digit zip code

ZipCode ID a0YQ0000002vg4 is ID for the record on the Zipcode Object for unknown ZipCodes

ZipCode ID a0YQ0000002wKD1  in my test class is the ID for the record on the Zipcode Object for Zip Code 98026. Would be better if this wasn't hard coded...but I got tired.

trigger getzip3 on Lead (before update, before insert) {
    //set for holding unique left5zip
    Set<String> left5zips = new Set<String>();    
    //get left5zip from each lead
    for (lead l : trigger.New) {
        String left5zip = l.left5zip__c;
        left5zips.add(left5zip);
    }
    Map<String<left5zip>, Id> zmap = new Map<String<left5zip>, Id> ();
    List<ZipCodeTable__c> zipcodeIdlist = [SELECT ZipCodeID__c, Id from ZipCodeTable__c WHERE ZipCodeID__c IN :left5zips];
    
    for (ZipCodeTable__c z : zipcodeIdlist) {
        zmap.put (z.ZipCodeID__c, z.Id); 
    }
       
   
    for (lead l : trigger.New) {
        String left5zip = l.left5zip__c;
        
        if (zmap.containsKey(left5zip)) {
            String key = zmap.get(l.left5zip__c);
            l.mikezip__c = key;
        }
        else {
            String key = 'a0YQ0000002vg4r';
            l.mikezip__c = key; 
        }
    }
   }

 This is the quick test class I wrote, and it passes with 100% coverage, but obviously that doesn't mean everything.

@isTest
private class TestZip {
    static testMethod void testZipChange() {
        lead[] leadtest = new lead[]{
         new lead(FirstName ='mike', LastName = 'Smith', Company = 'lolz', PostalCode = '98020'),
         new lead(FirstName ='fred', LastName = 'Smith', Company = 'lolz', PostalCode = '68516'),
         new lead(FirstName ='george', LastName = 'Smith', Company = 'lolz', PostalCode = '68517'),
         new lead(FirstName ='steve', LastName = 'Smith', Company = 'lolz', PostalCode = '98105'),
         new lead(FirstName ='mike', LastName = 'Smith', Company = 'lolz', PostalCode = '98106'),
         new lead(FirstName ='betty', LastName = 'Smith', Company = 'lolz', OwnerId = '0054000000131ru', PostalCode = '555555')
         };
         insert leadtest;
            
        Test.startTest();
        leadtest[0].PostalCode = '98026';
        update leadtest;
        Test.stopTest();
    
        leadtest = [SELECT id, PostalCode, mikezip__c FROM lead WHERE id IN:leadtest];
       
        system.assert(leadtest[0].mikezip__c == 'a0YQ0000002wKD1');
    }
}