bit_strings.rb

def bit_strings(index, array, result)
  # Base case
  if index == array.length
    result << array.dup
  else
    # Make a choice to include
    array[index] = 1
    bit_strings(index+1, array, result)
    # Make a choice to exclude
    array[index] = 0
    bit_strings(index+1, array, result)
  end
end

p bit_strings(0, [0,0,0], [])

Without dup, you will get same subarrays in the result array.