Count duplicates by using only single variable

count_duplicates.rb

class InterviewQuestions
  def initialize(count=0)
    @count = 0
  end

  def count_duplicates(arr)
    arr.sort! # your favorite sorting algo (n log n)
    arr.each_with_index {|index|
      if (index + 1 < arr.length)
        if (arr[index+1]==arr[index])
          @count +=1
        end
      end
    }
    @count
  end
end

describe InterviewQuestions do

  it "should find 0 duplicate numbers for empty array" do
    InterviewQuestions.new.count_duplicates([ ]).should == 0
  end

  it "should find 0 duplicate numbers for array with single element" do
    InterviewQuestions.new.count_duplicates([ 1 ]).should == 0
  end

  it "should find 4 duplicate numbers" do
    InterviewQuestions.new.count_duplicates([3, 1, 5, 9, 3, 2, 1, 1 , 4]).should == 4
  end

  it "should find 2 duplicate numbers for array including (+/-)" do
    InterviewQuestions.new.count_duplicates([ 5, 1 , -4, -9, -9]).should == 2
  end

  it "should find 0 duplicate numbers" do
    InterviewQuestions.new.count_duplicates([ 1, 7 , 2, 3, 4]).should == 0
  end

  it "should find 5 duplicate numbers" do
    InterviewQuestions.new.count_duplicates([ 1, 1, 1, 1, 1 ]).should == 5
  end
end