input: Write a python function to rotate elements of an array to the left.
  assert rotate_left([1, 2, 3, 4, 5])==[2, 3, 4, 5, 1]
  assert rotate_left([10, 20, 30, 40])==[20, 30, 40, 10]
  assert rotate_left([7, 8, 9])==[8, 9, 7]
output: def rotate_left(arr):
      if not arr:
          return arr
      return arr[1:] + arr[:1]