input: Write a function to find the smallest number in a list of numbers. If the list is empty, return None.
  def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest
  
  assert find_smallest_number([5, 4, 12, 17, 9])==4
  assert find_smallest_number([10, 20, 30, 40, 50])==10
  assert find_smallest_number([1, 1, 1, 1, 1])==1
  assert find_smallest_number([-5, -10, 0, 5, 10])==-10
  assert find_smallest_number([])==None
output: def find_smallest_number(numbers):
      if not numbers:
          return None
      smallest = numbers[0]
      for num in numbers:
          if num < smallest:
              smallest = num
      return smallest