find_duplicates.rb

class Array
  def find_duplicates(other_array)
    hsh = {}
    other_array.each{|val| hsh[val] = true}

    # Ya, could be done easier using Array.select{}, but that feels like cheating
    # return test_array.select{|val| hsh[val] == true}
    result_array = []
    self.each{ |val| result_array << val if hsh[val] }
    result_array
  end
end

array1 = [2, 8, 3, 4, 42, 58, 7, 83, 34, 72, 22, 47, 28, 92, 51]
array2 = [52, 59, 83, 34, 2, 76, 99, 48, 42, 4]

if (result1 = array1.find_duplicates(array2)) == [2, 4, 42, 83, 34]
  puts "Pass"
else
  puts "expected [2, 4, 42, 83, 34], got #{result1.inspect}"
end

if (result2 = array2.find_duplicates(array1)) == [83, 34, 2, 42, 4]
  puts "Pass"
else
  puts "expected [83, 34, 2, 42, 4], got #{result2.inspect}"
end