input: Write a function to find the largest number in a list.
  assert find_largest_number([7, 2, -4, 5, 8, 1]) == 8
  assert find_largest_number([3, 5, 9, 1, 6, 2]) == 9
  assert find_largest_number([-10, -3, -5, -1, -7]) == -1
output: def find_largest_number(lst):
      if not lst:
          return None
      largest = lst[0]
      for num in lst:
          if num > largest:
              largest = num
      return largest