input: Write a function to find the largest prime factor of a given number. If the number is less than 2, return None.
  assert largest_prime_factor(331)==331
  assert largest_prime_factor(13195)==29
  assert largest_prime_factor(600851475143)==6857
  assert largest_prime_factor(1)==None
  assert largest_prime_factor(0)==None
output: def largest_prime_factor(n):
      if n < 2:
          return None
      largest = None
      factor = 2
      while factor * factor <= n:
          while (n % factor) == 0:
              largest = factor
              n //= factor
          factor += 1
      if n > 1:
          largest = n
      return largest