trigger ContactCountOnAccount on Contact (after insert, after update, after delete) {
    Set<Id> AccountIds = new Set<Id>();
    List<Account> AccountsToUpdate = new List<Account>();
    for(Contact c : trigger.new){
        AccountIds.add(c.AccountId);
    }
    if(trigger.isUpdate || trigger.isdelete){
        for(Contact o : trigger.old){
            AccountIds.add(o.AccountId);
        }
    }
    map<Id, Account> MapAcc = new map<Id, account>([SELECT id, No_of_Contacts__c FROM Account WHERE Id IN : AccountIds]);
    List<Account> LstAcc = New List<Account>([SELECT Id, No_of_Contacts__c, (SELECT Id FROM Contacts) FROM Account WHERE Id IN :AccountIds]);
    for(Account acc : LstAcc){
        MapAcc.get(acc.id).No_of_Contacts__c = acc.contacts.size();
        AccountsToUpdate.add(MapAcc.get(acc.id));
    }
    update AccountsToUpdate;
}