On Contact to update the Account Name field with contact Last Name, concatenated in the end. Example: If you Create A Contact whose FirstName='Sarthak' and LastName='Saxena' and the Accountname ='Tata', Then Account name must become 'TataSaxena'. 
Update the above trigger to delete the Last Name from the Account field when a Contact is deleted without deleteing the other Last name.



trigger AddlastNmaeAccount on Contact (after insert,before delete) 
{
    if(trigger.isInsert)
    {
    set<id> s=new set<id>();
    list<Account> listaccount=new list<Account>();
    for(contact con:trigger.new)
    {
        s.add(con.accountId);
    }
    Map<id,account> mv=new Map<id,account>([select id,name from account where id IN:s]);
    for(contact cont:trigger.new)
    {
        Account a=new Account();
        a.Id=mv.get(cont.AccountId).id;
        a.name=mv.get(cont.AccountId).Name+cont.LastName;
        listaccount.add(a);
    }
    Update listaccount;
    }
    if(Trigger.isDelete)
    {
     set<id> s1=new set<id>();
     list<Account> listaccount=new list<Account>();
     for(Contact oContact : Trigger.Old)
     {
         if(oContact.lastName!= null) 
         s1.add(oContact.accountId);
     }
     Map<id,account> mv=new Map<id,account>([select id,name from account where id IN:s1]);
     for(contact cont:trigger.new)
     {
        Account a=new Account();
        a.Id=mv.get(cont.AccountId).id;
        a.name=a.name.replace(cont.LastName,a.name);

        listaccount.add(a);
     }
        update listaccount;
    }
}