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+#!/usr/bin/env perl
+# Author: Steven Wilson
+# Date: 2019-12-16
+# Week: 039
+
+# TASK #2
+# Contributed by Andrezgz
+# Write a script to demonstrate Reverse Polish notation(RPN). Checkout
+# the wiki page for more information about RPN.
+# https://en.wikipedia.org/wiki/Reverse_Polish_notation
+
+use strict;
+use warnings;
+use Test::More tests => 2;
+
+my %dt = (
+    '+' => sub { $_[0] + $_[1] },
+    '-' => sub { $_[0] - $_[1] },
+    'x' => sub { $_[0] * $_[1] },
+    '/' => sub { $_[0] / $_[1] },
+);
+
+ok( evaluate("1 2 +") == 3 );
+ok( evaluate("15 7 1 1 + - / 3 x 2 1 1 + + -") == 5 );
+
+# for each token in the postfix expression:
+#   if token is an operator:
+#     operand_2 ← pop from the stack
+#     operand_1 ← pop from the stack
+#     result ← evaluate token with operand_1 and operand_2
+#     push result back onto the stack
+#   else if token is an operand:
+#     push token onto the stack
+# result ← pop from the stack
+
+sub evaluate {
+    my $expression = shift;
+    my @expression = split " ", $expression;
+    my @stack      = ();
+    for my $token (@expression) {
+        if ( $token =~ /\d+/ ) {
+            push @stack, $token;
+        }
+        else {
+            my $operand_2 = pop @stack;
+            my $operand_1 = pop @stack;
+            my $result    = $dt{$token}->( $operand_1, $operand_2 );
+            push @stack, $result;
+        }
+    }
+    return pop @stack;
+}