use strict;
use Data::Dumper;

my %before =
    (
     a => _s(qw(c)),
     b => _s(qw(d e)),
     c => _s(qw(l)),
     d => _s(qw(e a)),
     e => _s(qw(c)),
     f => _s(qw(g d)),
     g => _s(qw(c)),
     h => _s(qw(g i)),
     i => _s(qw()),
     j => _s(qw(c)),
     k => _s(qw()),
     l => _s(qw()),
     n => _s(qw(c)),
     o => _s(qw()),
    );

print Dumper(\%before);

my @order = sort {
    #print "Checking $a vs $b\n";
    prereq($a, $b) <=> prereq($b, $a)
} keys %before;

print "@order\n";

sub prereq
{
    my ($l, $r) = @_;

    if (exists $before{$l}{$r})
    {
        return 1;
    }
    if (grep { prereq($_, $r) } keys %{$before{$l}})
    {
        return 1;
    }
    0;
}

# setup - create a hash for easy access
sub _s
{
    my %h = map { $_ => 1 } @_;
    \%h
}

##</code><code>##

l c e n a d j k g h b f o i