Add permutations exercise for recursion

recursion/4_permutations/.rspec

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+--require spec_helper --format documentation --color

recursion/4_permutations/exercises/permutation_exercises.rb

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+def permutations(array)
+  # Write a method that takes in an array of integers and returns an array of
+  # all possible permutations of the original array. The permutations of a set
+  # are the different ways the elements can be arranged.
+  #
+  # For simplicity, the integers are guaranteed to not repeat.
+  #
+  # Examples:
+  # `permutations([1, 2, 3])` has six different permutations (or ways the elements can be arranged)
+  #  it should return `[[1, 2, 3], [1, 3, 2], [2, 1, 3], [2, 3, 1], [3, 1, 2], [3, 2, 1]]`
+  #
+  # `permutations([])` returns `[[]]`, as there's only one arrangement of an empty set
+  #
+  # NOTE: the tests do not check for ordering, so a return of `[[1, 2], [2, 1]]`
+  # will be treated the same as `[[2, 1], [1, 2]]`
+end

recursion/4_permutations/spec/permutation_exercises_spec.rb

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+require 'spec_helper'
+require_relative '../exercises/permutation_exercises'
+
+RSpec.describe '#permutations' do
+  it "returns 1 possible permutation for a set containing 0 numbers" do
+    expect(permutations([])).to eq [[]]
+  end
+
+  xit "returns 2 possible permutations for a set containing 2 numbers" do
+    expect(permutations([1, 2])).to match_array([[2, 1], [1, 2]])
+  end
+
+  xit "returns 6 possible permutations for a set containing 3 numbers" do
+    expected_set = [
+      [1, 2, 3], [1, 3, 2], [2, 1, 3],
+      [2, 3, 1], [3, 1, 2], [3, 2, 1]
+    ]
+
+    expect(permutations([1, 2, 3])).to match_array(expected_set)
+  end
+
+  xit "returns 24 possible permutations for a set containing 4 numbers" do
+    expected_set = [
+      [1, 2, 3, 4], [1, 2, 4, 3], [1, 3, 2, 4], [1, 3, 4, 2],
+      [1, 4, 2, 3], [1, 4, 3, 2], [2, 1, 3, 4], [2, 1, 4, 3],
+      [2, 3, 1, 4], [2, 3, 4, 1], [2, 4, 1, 3], [2, 4, 3, 1],
+      [3, 1, 2, 4], [3, 1, 4, 2], [3, 2, 1, 4], [3, 2, 4, 1],
+      [3, 4, 1, 2], [3, 4, 2, 1], [4, 1, 2, 3], [4, 1, 3, 2],
+      [4, 2, 1, 3], [4, 2, 3, 1], [4, 3, 1, 2], [4, 3, 2, 1]
+    ]
+
+    expect(permutations([1, 2, 3, 4])).to match_array(expected_set)
+  end
+end

recursion/4_permutations/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

solutions/recursion/4_permutations/.rspec

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+--require spec_helper --format documentation --color

solutions/recursion/4_permutations/exercises/permutation_exercises.rb

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+def permutations(array, index = 0, results = [])
+  results << array.dup if index == array.length
+
+  (index...array.length).each do |i|
+    array[index], array[i] = array[i], array[index]
+    permutations(array, index + 1, results)
+    array[index], array[i] = array[i], array[index]
+  end
+
+  results
+end

solutions/recursion/4_permutations/spec/permutations_spec.rb

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+require 'spec_helper'
+require_relative '../exercises/permutation_exercises'
+
+RSpec.describe '#permutations' do
+  it "returns 1 possible permutation for a set containing 0 numbers" do
+    expect(permutations([])).to eq [[]]
+  end
+
+  it "returns 2 possible permutations for a set containing 2 numbers" do
+    expect(permutations([1, 2])).to match_array([[2, 1], [1, 2]])
+  end
+
+  it "returns 6 possible permutations for a set containing 3 numbers" do
+    expected_set = [
+      [1, 2, 3], [1, 3, 2], [2, 1, 3],
+      [2, 3, 1], [3, 1, 2], [3, 2, 1]
+    ]
+
+    expect(permutations([1, 2, 3])).to match_array(expected_set)
+  end
+
+  it "returns 24 possible permutations for a set containing 4 numbers" do
+    expected_set = [
+      [1, 2, 3, 4], [1, 2, 4, 3], [1, 3, 2, 4], [1, 3, 4, 2],
+      [1, 4, 2, 3], [1, 4, 3, 2], [2, 1, 3, 4], [2, 1, 4, 3],
+      [2, 3, 1, 4], [2, 3, 4, 1], [2, 4, 1, 3], [2, 4, 3, 1],
+      [3, 1, 2, 4], [3, 1, 4, 2], [3, 2, 1, 4], [3, 2, 4, 1],
+      [3, 4, 1, 2], [3, 4, 2, 1], [4, 1, 2, 3], [4, 1, 3, 2],
+      [4, 2, 1, 3], [4, 2, 3, 1], [4, 3, 1, 2], [4, 3, 2, 1]
+    ]
+
+    expect(permutations([1, 2, 3, 4])).to match_array(expected_set)
+  end
+end

solutions/recursion/4_permutations/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