How do I get value from parent row
May be this will help.

with topicdet as
(select t.topic_id as topic,c.conf as configuration,t.parent_topic_id as parenttopic from topic t left outer join config c on t.topic_id=c.topic_id)
select a.topic, NVL(a.configuration,CONNECT_BY_ROOT configuration) as conf from topicdet a
connect by nocycle prior a.topic=a.parenttopic;


if you dont want to use hierarchical query then

with topicdet as
(select t.topic_id as topic,c.conf as configuration,t.parent_topic_id as parenttopic from topic t left outer join config c on t.topic_id=c.topic_id)
select a.topic,case when a.configuration is null then (select b.configuration from topicdet b where b.parenttopic=a.topic) else a.configuration end as config from topicdet a