Actually, I felt like writing a little homework today :-) I leave the rest of prettyprinting to you. 
use warnings;
use strict;

my (%edges, %processed);

while (<DATA>) {
        if (/(\d+),(\d+)/) {
                $edges{$1} = [] unless (exists $edges{$1});
                push(@{$edges{$1}}, $2);
        }
};
close DATA;

for (sort keys %edges) {
        print_tree($_) unless exists $processed{$_};
}

sub print_tree {
        my ($node, $pp) = @_;
        $pp = '' unless defined $pp;
        print "$pp$node\n";
        $processed{$node} = 1;
        return unless exists $edges{$node};
        for (@{$edges{$node}}) {
                print "$pp$_ - cycle detected\n" if exists $processed{
+$_};
                print_tree($_, $pp.' ') unless exists $processed{$_};
        }
}
__DATA__
1,3
1,4
1,5
3,8
3,9
4,11
4,18
8,14
8,15
5,17
5,16
17,21

[download]

In reply to Re: representing a tree by pjotrik
in thread representing a tree by baxy77bax

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.