Flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

flatten_a_nesting_array.rb

# In Ruby, we can just use Array#flatten
#
# def flatten(array)
#   array.flatten
# end

# In case you want a manmade version
def flatten(array)
  array.reduce([]) do |res, element|
    Array === element ? res += flatten(element) : res << element
  end
end

array = [[1,2,[3]],4]
expected = (1..4).to_a
fail "Unequal" unless expected == flatten(array)

array = [[1, 2, [3], [4, 5, [6, 7]]], 8]
expected = (1..8).to_a
fail "Unequal" unless expected == flatten(array)