public int countPrimes(int n) {
        boolean[] isPrime = new boolean[n];
        for (int i = 2; i < n; ++i)
            isPrime[i] = true;
        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;
    }