use Sort::Topological;

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 = reverse
    Sort::Topological::toposort(
                                sub {
                                    @{$before{$_[0]}};
                                },
                                [ keys %before ],
                               );

print "@order\n";

sub _s
{
    [ @_ ]; #my %h = map { $_ => 1 } @_;
    #\%h
}

##</code><code>##

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