input: Write a python function to arrange a list of elements using the radix sort algorithm.
  assert radix_sort([15, 79, 25, 68, 37]) == [15, 25, 37, 68, 79]
  assert radix_sort([9, 11, 8, 7, 3, 2]) == [2, 3, 7, 8, 9, 11]
  assert radix_sort([36, 12, 24, 26, 29]) == [12, 24, 26, 29, 36]
output: def radix_sort(arr):
      if len(arr) == 0:
          return []
      max_num = max(arr)
      exp = 1
      while max_num // exp > 0:
          count = [0] * 10
          output = [0] * len(arr)
          for i in arr:
              count[(i // exp) % 10] += 1
          for i in range(1, 10):
              count[i] += count[i - 1]
          for i in reversed(arr):
              output[count[(i // exp) % 10] - 1] = i
              count[(i // exp) % 10] -= 1
          arr = output[:]
          exp *= 10
      return arr