Apex Trigger to Count Child records on Custom Object

trigger CaseMaster on Case (after delete, after insert, after undelete) { 
    set<string> TUWIDs = new set<string>();
    if(trigger.isInsert){
        for(Case TUW : Trigger.new){
                TUWIDs.add(TUW.Trucks_Under_Warranty__c);
                 }   
        }else if(trigger.isDelete){
            for(Case TUW : Trigger.old){   
                TUWIDs.add(TUW.Trucks_Under_Warranty__c);
            }
        }else if(trigger.isUnDelete){
            for(Case TUW : Trigger.new){
                TUWIDs.add(TUW.Trucks_Under_Warranty__c);
                    }
            }
            if(TUWIDs.size() > 0){
                CaseHelper ch = new CaseHelper();
                ch.updateCaseCount(TUWIDs);
            }
}

@isTest
public with sharing class CaseHelper { 
        public void updateCaseCount(set<string> CaseIDs){ 
            if(CaseIDs == null){
                return; 
            } 
            if(CaseIDs.size() == 0){
                return; 
            }
            list<Case> CaseList = new list<Case>(); 
            CaseList = [Select ID From Case Where Trucks_Under_Warranty__c = :CaseIDs]; 
                integer totalCount; 
                map<string, integer> CaseIDToCaseCountMap = new map<string, integer>();    
                for(Case TUW : CaseList){ 
                if(CaseIDToCaseCountMap.containsKey(TUW.Trucks_Under_Warranty__c)){ totalCount = CaseIDToCaseCountMap.get(TUW.Trucks_Under_Warranty__c); 
                }else{ 
                    totalCount = 0; 
                }
                totalCount += 1; 
                CaseIDToCaseCountMap.put(TUW.Trucks_Under_Warranty__c, totalCount); 
                    } 
            list<Case> CaseUpdateList = new list<Trucks_Under_Warranty__c>(); 
            Case  CaseTemp; 
            for(string CaseID : CaseIDToCaseCountMap.keySet()){ 
                CaseTemp = new Case (); 
                CaseTemp.ID = CaseID; 
                CaseTemp.Case_Count__c = quoteLineItemIDToAdditionalCostMap.get(CaseID); 
                CaseUpdateList.add(CaseTemp); 
            } 
            if(CaseUpdateList.size() > 0){ update CaseUpdateList; 
        } 
    } 
}