Display The Cumulative Count Of Contacts On The Account Object Using An Apex Trigger.

The trigger must be authored on the Contact object. The following code functions for various operations:

• Inserting a Contact with an Account
• Updating/Switching Accounts on Contacts (updating count on both Accounts)
• Removing an Account from a Contact
• Deleting a Contact
• Undeleting a Contact

trigger ContactTrigger on Contact (after insert,after update, after delete, after undelete) {
    
    if(Trigger.isAfter)
    {
        Map<Id,List<Contact>> MapOfActIdandConList = new Map<Id,List<Contact>>();
        List<Account> lstAct = new List<Account>();
        Set<Id> AccountIdSetNew = new Set<Id>();
        Set<Id> AccountIdSetOld = new Set<Id>();
        if(Trigger.isInsert || Trigger.isUndelete || Trigger.isUpdate)
        {
            for(Contact contNew : Trigger.New)
            {
                if(contNew.AccountId != null)
                {
                    AccountIdSetNew.add(contNew.AccountId);
                }
            }
            if(AccountIdSetNew.size()>0){
                List<Contact> lstContNew = [Select Id,AccountId from Contact where AccountId in: AccountIdSetNew ];
                for(Contact ct : lstContNew)
                {
                    if(MapOfActIdandConList.containsKey(ct.AccountId))
                    {
                        MapOfActIdandConList.get(ct.AccountId).add(ct);
                    }
                    else
                    {
                        MapOfActIdandConList.put(ct.AccountId, new List<Contact> {ct});
                    }
                }
                
            }
        }
        
        if(Trigger.isDelete || Trigger.IsUpdate)
        {
            for(Contact contOld : Trigger.Old)
            {
                if(contOld.AccountId != null)
                {
                    AccountIdSetOld.add(contOld.AccountId);
                }
            }
            if(AccountIdSetOld.size()>0){
                List<Contact> lstContOld = [Select Id,AccountId from Contact where AccountId in: AccountIdSetOld ];
                for(Contact ct : lstContOld)
                {
                    if(MapOfActIdandConList.containsKey(ct.AccountId))
                    {
                        MapOfActIdandConList.get(ct.AccountId).add(ct);
                    }
                    else
                    {
                        MapOfActIdandConList.put(ct.AccountId, new List<Contact> {ct});
                    }
                }
            }
        }
        
        for( Id actId : MapOfActIdandConList.keySet())
        {
            Account act = new Account();
            act.Id = actId;
            act.Count_Contact__c = MapOfActIdandConList.get(actId).size();
            lstAct.add(act);
        }
        if(lstAct.size()>0)
        {
            update lstAct;
        }
    }
    
}