input: Write a python function to find the minimum number in an array.
  assert find_min([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]) == 1
  assert find_min([10, 20, 30, 5, 15, 25]) == 5
  assert find_min([-1, -5, -3, -4, -2, -6]) == -6
output: def find_min(arr):
      if not arr:
          return None
      min_num = arr[0]
      for num in arr:
          if num < min_num:
              min_num = num
      return min_num