actually i created a custom field "contacts name" in account object. under a particular account if we create contacts they should get displayed in that custom field seperated by ",". similarly if we delete a contact, remaining group of contacts should be displayed under that particular account... i did as below but its not working..... can u correct it plz

trigger tgr_updating_contact_2 on Contact (after insert, after update, after delete) {
  set<id>accid = new set<id>();
    if(trigger.isdelete){
    for(contact c:trigger.old){
        accid.add(c.accountid);
    }
   }
    else{
        for(contact c:trigger.new){
        accid.add(c.accountid);
    }
   }
        list<account>accnt = new list<account>();
        accnt = [select id, contacts_name__c,(select firstname, lastname from contacts) from account where id in:accid];
        for(account a:accnt){
            a.contacts_name__c = null;
        for(contact c:a.contacts){
                if(a.contacts_name__c==null)
                    a.contacts_name__c = c.lastname;
                else
                    a.contacts_name__c = c.lastname+ ','+ a.contacts_name__c;
    }
        for(contact c:a.contacts){
                if(Trigger.isDelete && c.AccountId != null){
              a.contacts_name__c= a.contacts_name__c;
         }
       }
     }
        update accnt;
}