How to count numberofcontacts trigger

To Count numberofcontacts using trigger.We can use RollupSummary for master-detail  relationship,but we  can create  

only 2 master-detail  reationships  for an  object.

1.we  have  to  create  a read-only field with  number to get count

2.we should create read-only field  in parent object.

3.we  have  to get count from  the child  object,so   we  write   trigger only  child object.

Note:If any queries please comment.

trigger NumberofContacts on contact(after insert,after update,after delete)

{

map<id,integer> mapacc=new map<id,integer>();

list<Account> 1stacc=new list<Account>();

if(trigger.isInsert)

{

for(contact con:trigger.new)

{

integer count=0;

if(mapacc.containskey(con.Accountid))

{

count+=mapacc.get(con.accountid);

}

count++;

mapacc.put(con.Accountid,count);

}

if(!mapacc.isempty())

{

for(Account acc:[select id,name,numberofcontacts__c from Account where id in:mapacc.keyset()])

{

Account a:new Account(id=acc.id);

mapacc.containskey(acc.id);

if(a.numberofcontacts__c!=null)

{

a.numbeeofcontacts__c=acc.numberofcontacts__c+mapacc.get(acc.id);

}

else

{

a.numberofcontacts__c=mapacc.get(acc.id);

}

firstacc.add(a);

}

}

}

if(trigger.isUpdate)

{

for(contact con:trigger.new)

{

integer count=0;

if(mapacc.containskey(con.Accountid))

{

count+=mapacc.get(con.accountid);

}

count++;

mapacc.put(con.Accountid,count);

}

for(contact con:trigger.old)

{

integer count=0;

if(mapacc.containskey(con.Accountid))

{

count+=mapacc.get(con.accountid);

}

count--;

mapacc.put(con.Accountid,count);

}

if(!mapacc.isempty())

{

for(Account acc:[select id,name,numberofcontacts__c from Account where id in:mapacc.keyset()])

{

Account a:new Account(id=acc.id);

mapacc.containskey(acc.id);

if(a.numberofcontacts__c!=null)

{

a.numbeeofcontacts__c=acc.numberofcontacts__c+mapacc.get(acc.id);

}

else

{

a.numberofcontacts__c=mapacc.get(acc.id);

}

firstacc.add(a);

}

}

}

if(trigger.isDelete)

{

for(contact con:trigger.old)

{

integer count=0;

if(mapacc.containskey(con.Accountid))

{

count+=mapacc.get(con.accountid);

}

count--;

mapacc.put(con.Accountid,count);

}

if(!mapacc.isempty())

{

for(Account acc:[select id,name,numberofcontacts__c from Account where id in:mapacc.keyset()])

{

Account a:new Account(id=acc.id);

mapacc.containskey(acc.id);

if(a.numberofcontacts__c!=null)

{

a.numbeeofcontacts__c=acc.numberofcontacts__c+mapacc.get(acc.id);

}

else

{

a.numberofcontacts__c=mapacc.get(acc.id);

}

firstacc.add(a);

}

}

}

if(!firstacc.isempty())

           {

               update firstacc;

           }

}