input: Write a python function to find the number with the highest frequency in an array using the Counter class from the collections module.
  assert highest_frequency([1, 2, 3, 1, 2, 1]) == 1
  assert highest_frequency([4, 5, 6, 5, 6, 5, 7]) == 5
  assert highest_frequency([1, 1, 1, 1, 1, 1]) == 1
output: def highest_frequency(arr):
      from collections import Counter
      count = Counter(arr)
      return count.most_common(1)[0][0]