rain.pl

#!/usr/bin/env perl 

use strict;
use warnings;

#  +
#  +   +
# ++  ++
# ++ ++++
#++++++++

my @heights = qw(1 3 5 1 2 3 4 2);
my $mo = {
    left => {
        ind  => 0,
        next => 1
    },
    right => {
        ind  => -1,
        next => -1
    }
};
my ($min, $max)  = ('') x 2;
my $prev = 0;
my $sum  = 0;

for (@heights) {
    my $new_max;

    if ($heights[$mo->{left}->{ind}] > $heights[$mo->{right}->{ind}]) {
        $new_max = 'left';
        $min = 'right';
    } else {
        $new_max = 'right';
        $min = 'left';
    }
    if ($max ne $new_max) {
        $max  = $new_max;
        $prev = $heights[$mo->{$min}->{ind}];
    }

    if ($prev > $heights[$mo->{$min}->{ind}]) {
        $sum += $prev - $heights[$mo->{$min}->{ind}];
    }

    $mo->{$min}->{ind} += $mo->{$min}->{next};
}

print "Sum: $sum\n";