Add example implementation of parallel_letter_frequency exercise

Co-authored-by: KOTP <[email protected]>

exercises/practice/parallel-letter-frequency/.meta/example.rb

@@ -0,0 +1,23 @@
+class ParallelLetterFrequency
+  def self.count(texts)
+    ractors = (0...texts.length).map do |i|
+      Ractor.new(texts[i]) do |text|
+        text.downcase.each_grapheme_cluster.select do |cluster|
+          cluster.match?(/\p{Alpha}/)
+        end.tally
+      end
+    end
+
+    tally = Hash.new(0)
+
+    until ractors.empty?
+      ractor, result = Ractor.select(*ractors)
+      ractors.delete ractor
+      result.each do |key, value|
+        tally[key] += value
+      end
+    end
+
+    tally
+  end
+end

exercises/practice/parallel-letter-frequency/parallel_letter_frequency.rb

@@ -1,6 +1,8 @@
 =begin
-Write your code for the 'Parallel Letter Frequency' exercise in this file. Make the tests in
-`parallel_letter_frequency_test.rb` pass.
+Write your code for the 'Parallel Letter Frequency' exercise in this file. Make
+the tests in `parallel_letter_frequency_test.rb` pass.
+
 To get started with TDD, see the `README.md` file in your
 `ruby/parallel_letter_frequency` directory.
 =end
+