Trigger for Count Number of Contacts Related To Account in Salesforce

trigger CountOfContactsRelatedToAccount on 

                Contact (after insert, after delete, after Undelete) {

    Set<Id> accId = new Set<Id>();

    if(Trigger.isInsert || Trigger.isUndelete){

        for(Contact con : Trigger.new){

            accId.add(con.AccountId);

        }

    }

    if(Trigger.isDelete){

        for(Contact con : Trigger.old){

            accId.add(con.AccountId);

        }

    }

    List<Account> accList = [Select Id,Name,Count_of_Contacts__c ,

(Select id from contacts) from Account where Id IN : accId];

    for(Account acc :accList){

        acc.Count_of_Contacts__c = acc.contacts.size();

    }

    update accList;

}