This is the answer to an old interview question I used to ask.

array_flattener.rb

class ArrayFlattener
    # simple recursive flatten
    def flatten(a)
        new_a = []
        a.each do |i|
            if i.instance_of?(Array)
                new_a.concat(flatten(i))
            else
                new_a << i    
            end
        end    
        new_a 
    end


    # slightly less simple non-recursive flatten
    def flatten_non_recursive(a)
        new_a = []
        # keep spinning while array not empty
        while !a.empty? 
            # array element
            if a.first.instance_of?(Array)
                # remove the element and place each element at front and resume from the new start
                a.shift.each_with_index do |element, j|
                    a.insert(j, element)
                end
            # non-array element, remove and place into new array
            else
                new_a << a.shift
            end
        end
        new_a
    end
end

require "test/unit"
class ArrayFlattenerTest < Test::Unit::TestCase
    def test_flatten_simple_case
        a = [1, 2, 3, 4]
        a2 = ArrayFlattener.new.flatten(a)
        assert_equal [1, 2, 3, 4], a2
    end

    def test_flatten_complex_case
        a = [1, 2, [3, [4]]]
        a2 = ArrayFlattener.new.flatten(a)
        assert_equal [1, 2, 3, 4], a2
    end

    def test_flatten_non_recursive_simple_case
        a = [1, 2, 3, 4]
        a2 = ArrayFlattener.new.flatten_non_recursive(a)
        assert_equal [1, 2, 3, 4], a2
    end

    def test_flatten_non_recursive_complex_case
        a = [1, 2, [3, [4]]]
        a2 = ArrayFlattener.new.flatten_non_recursive(a)
        assert_equal [1, 2, 3, 4], a2
    end
end