input: Write a function to swap the elements of two arrays of integers. The function should ensure that the two arrays are of the same length and then swap their elements at corresponding positions.
  assert swap([3, 5, 6, 9], [2, 4, 7, 8]) == None
  assert swap([3, 5, 6, 9], [2, 4, 7, 8]) == None
  assert swap([3], [2]) == None
output: def swap(arr1, arr2):
      if len(arr1) != len(arr2):
          return None
      for i in range(len(arr1)):
          arr1[i], arr2[i] = arr2[i], arr1[i]