Solution to Challenge manwar#1, Week 014

Author: rvandam

challenge-014/rob-van-dam/perl5/ch-1.pl

@@ -0,0 +1,40 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+use v5.10;
+
+my $M = shift @ARGV || 10;
+
+#slow();
+fast();
+
+# this works but is very slow for large $M
+# as its roughly O(n**2)
+sub slow {
+    my @a = (0);
+    for my $n (0..$M) {
+        my $r = 0;
+        foreach my $m (0..($n-1)) {
+            last if ! defined $a[$n];
+            $r = $n - $m if $a[$m] == $a[$n];
+        }
+        $a[$n + 1] = $r;
+        say $a[$n];
+    }
+}
+
+# this is much faster for large $M
+# by caching the last time we say a given number
+# so its roughly O(n**2) but requires roughly O(2n) space
+sub fast {
+    my @a = (0);
+    my @last = ();
+    for my $n (0..$M) {
+        my $m = $last[ $a[$n] ];
+        my $r = defined $m ? $n - $m : 0;
+        $last[$a[$n]] = $n;
+        $a[$n + 1] = $r;
+        say $a[$n];
+    }
+}