input: Write a python function to find the maximum subarray sum.
  assert maxSubarrySum([1, -2, 3, 4, -5]) == 7
  assert maxSubarrySum([-1, -2, -3, -4]) == 0
  assert maxSubarrySum([2, 1, -3, 4, -1, 2, 1, -5, 4]) == 6
output: def maxSubarrySum(arr):
      max_sum = 0
      current_sum = 0
      for num in arr:
          current_sum = max(0, current_sum + num)
          max_sum = max(max_sum, current_sum)
      return max_sum