Splitting an array between an integer for a quicksort method

partition.rb

def partition(array, pivot)
  return [], [] if array.empty?
  j = 0
  for i in 0...array.length
    if array[i] <= pivot
      array[j], array[i] = array[i], array[j]
      j += 1
    end
  end
  return array[0...j], array[j..-1]
end