input: Write a function to find the largest number in a list.
  assert find_largest([1, 7, 3, 4, 9, 2]) == 9
  assert find_largest([10, 20, 4, 45, 99, 23]) == 99
  assert find_largest([-1, -3, -10, -2]) == -1
output: def find_largest(numbers):
      if not numbers:
          return None
      largest = numbers[0]
      for number in numbers[1:]:
          if number > largest:
              largest = number
      return largest