input: Write a python function to find the largest number in an array.
  assert array_sum([1, 2, 3, 4, 5]) == 5
  assert array_sum([10, 20, 30, 40]) == 40
  assert array_sum([-1, -2, -3, -4]) == -1
output: def array_sum(arr):
      if not arr:
          return None
      largest = arr[0]
      for num in arr:
          if num > largest:
              largest = num
      return largest