input: def largest_prime_factor(n: int) -> int:
      """
      Find the largest prime factor of a given number.
      >>> largest_prime_factor(331)
      331
      >>> largest_prime_factor(13195)
      29
      """
output:     if n < 2:
          return None
      largest_factor = None
      for i in range(2, int(n**0.5) + 1):
          while n % i == 0:
              largest_factor = i
              n //= i
      if n > 1:
          largest_factor = n
      return largest_factor