input: Write a function to find the largest number in a given list. If the list is empty, return None.
  assert find_largest([1000, 506, 299, 620, 35])==1000
  assert find_largest([7, 3, 9, 5])==9
  assert find_largest([4])==4
  assert find_largest([])==None
output: def find_largest(lst):
      if not lst:
          return None
      return max(lst)