input: Write a python function to find the largest number in a list.
  assert find_largest([3, 6, 8, 11, 12]) == 12
  assert find_largest([1, 2, 3, 4, 5]) == 5
  assert find_largest([-10, -20, -30, -40]) == -10
output: def find_largest(lst):
      if not lst:
          return None
      largest = lst[0]
      for num in lst:
          if num > largest:
              largest = num
      return largest