input: Write a python function to find the largest number in a list.
  assert find_largest_number([1, 2, 3, 4, 5]) == 5
  assert find_largest_number([-1, -2, -3, -4]) == -1
  assert find_largest_number([10, 20, 30, 40, 50]) == 50
output: def find_largest_number(lst):
      if not lst:
          return None
      largest = lst[0]
      for number in lst:
          if number > largest:
              largest = number
      return largest