class Solution {
    public int countPrimes(int n) {
        if (n == 0) {
            return 0;
        }

        boolean[] notPrime = new boolean[n];
        int cnt = 0;
        final int limit = (int)Math.sqrt(n);

        for(int i = 2; i < n; ++i) {
            if(!notPrime[i]) {
                ++cnt;

                if (i <= limit) {
                    for(int j = 2; j * i < n; ++j) {
                        notPrime[i*j] = true;
                    }
                }
            }
        }

        return cnt;
    }
}