sub display1 {
    my $p = $_[0];
    return if !$p;
    display1($p->{'next'});
    print $p->{'value'}, ' ';
}

sub display2 {
    my $p = $_[0];
    my @stack;
    while ($p) {
        push @stack, $p;
        $p = $p->{'next'};
    }
    while ($p = pop @stack) {
        print $p->{'value'}, ' ';
    }
}