Sieve of Eratosthenes — All Primes ≤ N

boolean[] sieve(int n) {
  boolean[] isPrime = new boolean[n + 1];
  Arrays.fill(isPrime, true);
  isPrime[0] = isPrime[1] = false;
  for (int i = 2; (long)i * i <= n; i++)
    if (isPrime[i])
      for (int j = i * i; j <= n; j += i) isPrime[j] = false;
  return isPrime;
}