Oracle Hierarchical query: how to follow max on each level
This seems to solve the question asked. Not sure if there is a solution without aggregating first:

select lpad(' ',2*(level-1)) || child as result
from 
  ( select parent, max(child) as child 
    from test_connect_by 
    group by parent
  ) t 
start with parent is null 
connect by prior child = parent
order by level ;