How to solve the array index out of bound exception .

replace your main class with my class.

public class CampaignAccountHelper {
    public void updateAndInsertOpportunity(List<CampaignAccount__c> campaignList)
    {
        List<CampaignAccount__c> lstCampAcc = new List<CampaignAccount__c>();    
        Map<Id,List<Opportunity>> mapOppRec = new Map<Id,List<Opportunity>>();
        List<opportunity> lstOpp = new List<opportunity>();
        List<Opportunity> newOpp = new List<Opportunity>();
        For(Account acc :[SELECT id,(SELECT id,Name,CampaignCount__c FROM Opportunities) FROM Account]){
            mapOppRec.put(acc.Id,acc.Opportunities);
        }        
        for(CampaignAccount__c camp :campaignList){
            if(camp.Account__c != NULL){
                lstCampAcc.add(camp); 
            }
        }     
        if(mapOppRec.size() > 0){
        for(CampaignAccount__c camAcc : lstCampAcc){  
            for(Opportunity opp : mapOppRec.get(camAcc.Account__c)){
                if(mapOppRec.containskey(camAcc.Account__c) && camAcc.Name == opp.Name){
              opp.CampaignCount__c = opp.CampaignCount__c != null ? opp.CampaignCount__c+1 : '0'; 
            lstOpp.add(opp);
                }   
            }
          }
        } 
       system.debug('lstOpp============> ' + lstOpp);
        if(lstOpp.size() > 0){
        update lstOpp;     
        }
    }
}