input: Write a function to identify the largest number in a given list. If the list is empty, return None.
  def find_max_number(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num
  
  assert find_max_number([1000, 506, 299, 620, 35])==1000
  assert find_max_number([10, 20, 30, 40, 50])==50
  assert find_max_number([])==None
  assert find_max_number([-1, -5, -10, -2])==-1
  assert find_max_number([0])==0
output: def find_max_number(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num