input: Write a python function to find the maximum product of two distinct elements in an array.
  assert max_product_of_two([1, 2, 3, 4, 5]) == 20
  assert max_product_of_two([5, 7, 1, 2, 9, 8]) == 72
  assert max_product_of_two([0, -1, -3, -7, -5]) == 15
output: def max_product_of_two(arr):
      if len(arr) < 2:
          return None
      arr.sort()
      return max(arr[-1] * arr[-2], arr[0] * arr[1])