input: Write a function to find the largest number in the given list. If the list is empty then return None.
  assert largest_number([1000, 506, 299, 620, 35])==1000
  assert largest_number([-5, -2, -10, -1])==-1
  assert largest_number([10])==10
  assert largest_number([])==None
output: def largest_number(lst):
      if not lst:
          return None
      return max(lst)