with 
 t as (select level i, cast(level as varchar2(1)) c from dual connect by level<=&d)
,x(l,s,n) as (
       select 1 l, c s, chr(97)||c||' ' from t
       union all
       select l+1, x.s||t.c, n||chr(98+l)||i||' '
       from x
            join t
                 on instr(s,c)=0
                    and not exists(select 0 from dual 
                                   where L+1 - t.i = level - substr(s,level,1)
                                      or L+1 + t.i = level + substr(s,level,1)
                                   connect by level<=length(s))
       where L<&d
 )
select n
from x
where l=&d