2020_5.pl

my $max_seat = undef;
my $min_seat = undef;
my %exist=();

while( my $line = <STDIN> ) {
        chomp( $line );
        $line =~ tr/BRFL/1100/;
        my $ans = oct( "0b".$line );
        $exist{$ans}=1;
        $min_seat=$ans if( !defined( $min_seat ) || $ans < $min_seat );
        $max_seat=$ans if( !defined( $max_seat ) || $ans > $max_seat );
}

print "Max: $max_seat\n";

my $ct=$min_seat+1;
while( $ct < $max_seat ) {
        if( exists( $exist{$ct-1} ) && !exists( $exist{$ct} ) && exists( $exist{$ct+1} ) ) {
                print "SEAT: $ct\n";
        }
        $ct++;
}