Recursion Exercises: Add new factorial exercise

recursion/1_factorial/.rspec

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+--require spec_helper

recursion/1_factorial/exercises/factorial.rb

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+def factorial(num)
+  # write a recursive method for calculating the [factorial](https://simple.wikipedia.org/wiki/Factorial) of a number
+end

recursion/1_factorial/spec/factorial_spec.rb

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+require "spec_helper"
+require_relative "../exercises/factorial"
+
+RSpec.describe "Factorial" do
+  describe "factorial exercise" do
+    it "returns 1 when passed the number 1" do
+      expect(factorial(1)).to eq(1)
+    end
+
+    xit "returns 24 when passed the number 4" do
+      expect(factorial(4)).to eq(24)
+    end
+
+    xit "returns 3628800 when passed the number 10" do
+      expect(factorial(10)).to eq(3628800)
+    end
+
+    xit "returns 1 when passed the number 0" do
+      expect(factorial(0)).to eq(1)
+    end
+
+    xit "returns nil when passed a negative number" do
+      expect(factorial(-5)).to be_nil
+    end
+  end
+end

recursion/1_factorial/spec/spec_helper.rb

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+RSpec.configure do |config|
+  config.expect_with :rspec do |expectations|
+    expectations.include_chain_clauses_in_custom_matcher_descriptions = true
+  end
+
+  config.mock_with :rspec do |mocks|
+    mocks.verify_partial_doubles = true
+  end
+
+  config.shared_context_metadata_behavior = :apply_to_host_groups
+end
+
+module FormatterOverrides
+  def dump_pending(_)
+  end
+end
+
+RSpec::Core::Formatters::DocumentationFormatter.prepend FormatterOverrides