Add example implementation of parallel_letter_frequency exercise

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

@@ -0,0 +1,26 @@
+class ParallelLetterFrequency
+  def self.count(texts)
+    rs = (0...texts.length).map do |i|
+      Ractor.new(texts[i]) do |text|
+        res = Hash.new(0)
+        text.downcase.each_grapheme_cluster do |c|
+          res[c] += 1 if c.match?(/\p{Alpha}/)
+        end
+
+        res
+      end
+    end
+
+    tally = Hash.new(0)
+
+    until rs.empty?
+      r, v = Ractor.select(*rs)
+      rs.delete r
+      v.each do |k, v|
+        tally[k] += v
+      end
+    end
+
+    tally
+  end
+end