input: Write a python function to find all prime numbers up to a given number using the Sieve of Eratosthenes algorithm.
  assert find_primes(10) == [2, 3, 5, 7]
  assert find_primes(20) == [2, 3, 5, 7, 11, 13, 17, 19]
  assert find_primes(30) == [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
output: def find_primes(n):
      if n < 2:
          return []
      sieve = [True] * (n + 1)
      sieve[0] = sieve[1] = False
      for start in range(2, int(n**0.5) + 1):
          if sieve[start]:
              for multiple in range(start*start, n + 1, start):
                  sieve[multiple] = False
      return [num for num, is_prime in enumerate(sieve) if is_prime]