input: Write a python function to find the most common element in an array.
  assert find_common_element([1, 2, 2, 3, 3, 3, 4]) == 3
  assert find_common_element([5, 5, 5, 1, 1]) == 5
  assert find_common_element([-1, -2, 2, 5, 2, -3, 0, 1]) == 2
output: def find_common_element(arr):
      from collections import Counter
      if not arr:
          return None
      count = Counter(arr)
      return count.most_common(1)[0][0]