input: Write a python function to find the maximum sum of a sub-array.
  assert max_sum_subarray([2, -1, 3, 5, -7, 3]) == 8
  assert max_sum_subarray([-1, -2, -3, -4]) == 0
  assert max_sum_subarray([1, 2, 3, 4, 5]) == 15
output: def max_sum_subarray(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