This took me a little while to figure out how to solve, but it's really very easy. While there are still items to order, look for items that come before only items that have already been ordered. Insert them at start (or at end and reverse, like I did). The algorithm is at worst case about (N^2) / 2, efficient enough for the task given here.
use strict;
use warnings;
my %before = (
a => ['c'],
b => ['d','e'],
c => ['l'],
d => ['e','a'],
e => ['c'],
f => ['g','d'],
g => ['c'],
h => ['g','i'],
i => [],
j => ['c'],
k => [],
l => [],
n => ['c'],
o => [],
);
my ($k, $m, %p, @order);
while (scalar keys %before) {
for $k (keys %before) {
$m = 1;
for (@{$before{$k}}) {
if (!exists $p{$_}) {
$m = 0; last;
}
}
if ($m) {
push(@order, $k);
$p{$k} = ();
delete($before{$k});
}
}
}
@order = reverse @order;
print @order;
-
Are you posting in the right place? Check out Where do I post X? to know for sure.
-
Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
<u> <ul>
-
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
intervention).
-
Want more info? How to link
or How to display code and escape characters
are good places to start.