Day 11: recursive solution

diff --git a/2025/22alt.pl b/2025/22alt.pl
new file mode 100755 (executable)
index 0000000..1ee1b3d
--- /dev/null
+++ b/2025/22alt.pl
@@ -0,0 +1,16 @@
+#!/usr/bin/perl -w
+
+use v5.42;
+
+my %g = map { my ($n, @x) = /\w+/g; $n => \@x } <>;
+
+sub walk($node, $goal, $seen = {}) {
+       return 1 if $node eq $goal;
+       return $seen->{$node} if defined $seen->{$node};
+       $seen->{$node} += walk($_ => $goal, $seen)
+               for $g{$node}->@*;
+       return $seen->{$node} //= 0;
+}
+
+say walk('svr' => 'fft') * walk('fft' => 'dac') * walk('dac' => 'out')
+  + walk('svr' => 'dac') * walk('dac' => 'fft') * walk('fft' => 'out');