input: Write a function to shuffle an array of integers. The function should return the shuffled array.
  assert shuffle([1, 5, 9, 4, 2]) != [1, 5, 9, 4, 2]
  assert len(shuffle([1, 5, 9, 4, 2])) == 5
  assert set(shuffle([1, 5, 9, 4, 2])) == set([1, 5, 9, 4, 2])
  assert shuffle([]) == []
output: import random
  
  def shuffle(arr):
      random.shuffle(arr)
      return arr