input: Write a python function to find the prime factors of a given number.
  assert prime_factors(38) == [2, 19]
  assert prime_factors(100) == [2, 2, 5, 5]
  assert prime_factors(77) == [7, 11]
output: def prime_factors(n):
      i = 2
      factors = []
      while i * i <= n:
          if n % i:
              i += 1
          else:
              n //= i
              factors.append(i)
      if n > 1:
          factors.append(n)
      return factors