http://jagannathvalaiyapathy.blogspot.in/search/label/Salesforce%20Certification#.UUXktErdWho

countofcontacts

trigger Countofcontacts on Contact (after insert,after delete)
{
   set<id> accid=new set<id>();
   list<contact> contactlist=new list<contact>();
   list<contact> listcon=new list<contact>();
   list<account> acclist=new list<account>();
   list<account> listAcc=new list<account>();
    map<id,integer> mapCount=new map<id,integer>();
     if(trigger.isinsert)
     {
         for(contact con:trigger.new)
          {
             accid.add(con.accountid);
           }
      }
       if(trigger.isdelete)
      {
         for(contact con:trigger.old)
           {
              accid.add(con.accountid);
           }
      }
acclist=[select id,name from account where id in:accid];
contactlist=[select id,name,accountid from contact where accountid in:accid];
for(account acc:acclist)
{
    listcon.clear();
    for(contact c:contactlist)
    {
      if(c.accountid==acc.id)
      {
        listcon.add(c);
        mapCount.put(c.accountid,listcon.size());
       }
    }
}
if(acclist.size()>0)
{
    for(Account a:acclist)
     {
        if(mapCount.get(a.id)==null)
        a.Count_Of_Contacts__c=0;
          else
               a.Count_Of_Contacts__c=mapCount.get(a.id);
       listAcc.add(a);
     }
}
if(listAcc.size()>0)
update listAcc;
}