polynomial.rb

class Polynomial
  
  def initialize(coefs)
    if coefs.nil? || !coefs.kind_of?(Array) || coefs.size < 2
      raise ArgumentError, "Need at least 2 coefficients."
    end

    @coefs = coefs
  end
  
  def to_s
    power = @coefs.size
    parts = @coefs.map do |coef|
      power -= 1
      coef   = coef.to_i
      unless coef == 0
        part   = (power == 0 || coef.abs != 1) ? coef.to_s : (coef > 0 ? '' : '-')
        part  += (power == 0) ? '' : (power == 1) ? 'x' : "x^#{power}"
      end
    end.compact
    
    return parts.empty? ? '0' : (parts.compact * '+').gsub('+-', '-')
  end
  
end

polynomial_test.rb

# assuming that your solution is polynomial.rb
require 'test/unit'
require 'test/unit/ui/console/testrunner'
require 'polynomial'
 
class PolynomialTest < Test::Unit::TestCase
  def test_first_negative
    assert_poly("-3x^4-4x^3+x^2+6", -3, -4, 1, 0, 6) 
  end
 
  def test_simple
    assert_poly("x^2+2", 1, 0, 2) 
  end
 
  def test_first_minus_one
    assert_poly("-x^3-2x^2+3x", -1, -2, 3, 0)
  end
 
  def test_all_zero
    assert_poly("0", 0, 0, 0)
  end
 
  def test_last_one
    assert_poly("x-1", 1, -1)
  end
 
  def test_error
    e = assert_raise(ArgumentError) { Polynomial.new([1]) }
    assert_match(/Need at least 2 coefficients/, e.message)
  end
  
  def assert_poly(result, *c)
    assert_equal(result, Polynomial.new(c).to_s)
  end
end
 
Test::Unit::UI::Console::TestRunner.run(PolynomialTest)