input: Write a function to find the largest number in a list.
  assert largest_num_in_list([2, 5, 8, 10, 21]) == 21
  assert largest_num_in_list([-1, -5, -3, -4]) == -1
  assert largest_num_in_list([100, 200, 300, 400]) == 400
output: def largest_num_in_list(numbers):
      if not numbers:
          return None
      largest = numbers[0]
      for num in numbers:
          if num > largest:
              largest = num
      return largest