204. Count Primes

除了C語言課本里方法，另一種求質數的算法：
https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
給出的參考代碼：

public int countPrimes(int n) {
   boolean[] isPrime = new boolean[n];
   for (int i = 2; i < n; i++) {
      isPrime[i] = true;
   }
   // Loop's ending condition is i * i < n instead of i < sqrt(n)
   // to avoid repeatedly calling an expensive function sqrt().
   for (int i = 2; i * i < n; i++) {
      if (!isPrime[i]) continue;
      for (int j = i * i; j < n; j += i) {
         isPrime[j] = false;
      }
   }
   int count = 0;
   for (int i = 2; i < n; i++) {
      if (isPrime[i]) count++;
   }
   return count;
}