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#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use utf8;
my %op = ( '+' => sub { $_[0] + $_[1] },
'−' => sub { $_[0] - $_[1] },
'×' => sub { $_[0] * $_[1] },
'÷' => sub { $_[0] / $_[1] });
sub rpn {
my ($input) = @_;
my @tokens = split ' ', $input;
my @stack;
for my $token (@tokens) {
if ($token =~ /^[0-9]+$/) {
push @stack, $token;
} else {
push @stack, $op{$token}->(((pop @stack) // die 'Stack empty'),
(pop @stack) // die 'Stack empty');
}
}
my $result = pop @stack;
die "Left on stack: @stack" if @stack;
return $result
}
my $input = '15 7 1 1 + − ÷ 3 × 2 1 1 + + −';
my $expected_result = 5;
my $result = rpn($input);
die unless $result == $expected_result;
say $result;
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