trigger updateContactDetails on Opportunity(after insert, after update, after delete, after undelete){
    Set<Id> contactIdSet = new Set<Id>();
    if(trigger.isInsert || trigger.isUpdate || trigger.isUndelete){
        for(Opportunity opp : trigger.new){
            if(trigger.isInsert || trigger.isUndelete){
                if(opp.ContactId != null){
                    contactIdSet.add(opp.ContactId);
                }
            }
            else if(trigger.isUpdate && (trigger.oldMap.get(opp.id).ContactId != opp.ContactId || trigger.oldMap.get(opp.id).Credit__c != opp.Credit__c || trigger.oldMap.get(opp.id).Debit__c != opp.Debit__c)){
                if(opp.ContactId != null){
                    contactIdSet.add(opp.ContactId);
                }
                if(trigger.oldMap.get(opp.id).ContactId != null){
                    contactIdSet.add(trigger.oldMap.get(opp.id).ContactId);
                }
            }
        }
    }
    else if(trigger.isDelete){
        for(Opportunity opp : trigger.old){
            if(opp.ContactId != null){
                contactIdSet.add(opp.ContactId);
            }
        }
    }
    if(contactIdSet.size() > 0){
        List<Contact> listOfContactsToUpdate = new List<contact>();
        
        for(Contact con : [Select RDebit__c, RCredit__c, (Select Debit__c, Credit__c from Opportunities__r) from Contact where ID IN :contactIdSet]){
            con.RDebit__c = 0;
            con.RCredit__c = 0;
            
            for(Opportunity childOpp : con.Opportunities__r){
                if(childOpp.Debit__c != null){
                    con.RDebit__c += childOpp.Debit__c;
                }
                if(childOpp.Credit__c != null){
                    con.RCredit__c += childOpp.Credit__c;
                }
            }
            listOfContactsToUpdate.add(con);
        }
        
        if(listOfContactsToUpdate.size() > 0){
            update listOfContactsToUpdate;
        }
    }
}
//Take care of the compilation issues