with t as
( select 'A' parent,'B' child from dual UNION  
  select 'A' parent,'D' child from dual UNION
  select 'A' parent,'F' child from dual UNION
  select 'B' parent,'T' child from dual UNION
  select 'B' parent,'S' child from dual UNION
  select 'D' parent,'H' child from dual UNION
  select 'F' parent,'M' child from dual UNION
  select 'N' parent,'B' child from dual UNION
  select 'N' parent,'L' child from dual UNION
  select 'N' parent,'Z' child from dual UNION
  select 'L' parent,'D' child from dual UNION
  select 'L' parent,'C' child from dual UNION
  select 'Z' parent,'D' child from dual UNION
  select 'Z' parent,'F' child from dual 
 )
select distinct parent from t
 start with child='D'
connect by child = prior parent;