input: Write a python function to find the maximum element in an array.
  assert maxElement([5, 25, 10, 9]) == 25
  assert maxElement([-1, -2, -3, -4, 0]) == 0
  assert maxElement([100, 200, 300, 400]) == 400
output: def maxElement(arr):
      if not arr:
          return None
      max_val = arr[0]
      for num in arr:
          if num > max_val:
              max_val = num
      return max_val