A --- B
| |
| |
| |
D --- C
##```
##
A----B
| / |
| / |
|/ |
D----C
##
```

```
##
for my $node ($graph->nodes) {
my @paths = $graph->find_shortest_paths_starting_from($node);
# Now, suppose @paths is an array containing arrays of
# nodes that are the actual nodes on the shortest paths,
# with the first being $node and the last being the end of
# the path. For instance, [ $node, $other_node, ..., $end_node ]
# for the shortest path from $node to $end_node.
# Next, for each endpoint, try to find a path back, but
# remove the nodes from consideration that are on the
# known shortest path. This is to prevent the algorithm
# from re-discovering the shortest path, and will ensure
# that the two possible found paths are mutually exclusive,
# i.e. do not contain same nodes. (Otherwise you would have
# an even shorter cycle... could this be used for our benefit?)
for my $path (@paths) {
my $new_graph = $graph->copy;
# Slice away the first and last nodes, as they are still of
# interest to us.
$new_graph->remove_nodes(@$path[1 .. $#{@$path}-1]);
my @paths_back = $new_graph->find_shortest_paths_starting_from($path->[-1]);
for my $path_back (@paths_back) {
if ($path_back->[-1] eq $node) {
print "We have a cycle involving $node!\n";
}
}
}
}
```