trigger UpdateContactCountOnAccount on Contact (after insert, after update, after delete)
{
    Set<Id> AccountIds = new Set<Id>();

    if(!Trigger.isDelete)
    {
        for (Contact ct : Trigger.new)
        {
            if(Trigger.isInsert && ct.AccountId != null)
            {
                AccountIds.add(ct.AccountId);
            }
            if(Trigger.isUpdate)
            {
                if(ct.AccountId==null && Trigger.oldMap.get(ct.Id).AccountId != null)
                {
                    AccountIds.add(Trigger.oldMap.get(ct.Id).AccountId);
                }
                if(ct.AccountId!=null && Trigger.oldMap.get(ct.Id).AccountId != null
                                                                    && ct.AccountId != Trigger.oldMap.get(ct.Id).AccountId)
                {
                    AccountIds.add(ct.AccountId);
                    AccountIds.add(Trigger.oldMap.get(ct.Id).AccountId);
                }
                if(ct.AccountId!=null && Trigger.oldMap.get(ct.Id).AccountId == null)
                {
                    AccountIds.add(ct.AccountId);
                }
            }
        }
    }
    else
    {
        for (Contact ct : Trigger.old)
        {
            if(Trigger.isDelete && ct.AccountId != null)
            {
                AccountIds.add(ct.AccountId);
            }
        }
    }

    List<Account> AcctToUpdate = new List<Account>();
    for (AggregateResult ar: [Select Count(Id) ContactCount, AccountId
                              from Contact where AccountId IN: AccountIds GROUP BY AccountId])
    {
        Account tmp = new Account(Id=(Id)ar.get('AccountId'), No_of_Contacts__c=(Decimal)ar.get('ContactCount'));
        AcctToUpdate.add(tmp);
    }
    if(AcctToUpdate.size()>0)
        update AcctToUpdate;
 }