input: Write a function to find the largest number in the given list. If the list is empty, return None.
  assert largest_number([1000, 506, 299, 620, 35])==1000
  assert largest_number([-1, -5, -3, -2])==-1
  assert largest_number([0, 0, 0, 0])==0
  assert largest_number([1])==1
  assert largest_number([])==None
output: def largest_number(lst):
      if not lst:
          return None
      return max(lst)