input: Write a function to find the largest prime factor of a given number. If the given number is less than 2, return None.
  assert largest_prime_factor(331)==331
  assert largest_prime_factor(100)==5
  assert largest_prime_factor(7)==7
  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
      for i in range(2, int(n**0.5) + 1):
          while n % i == 0:
              largest = i
              n //= i
      if n > 1:
          largest = n
      return largest