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

        bool notPrime[n] = {false};
        int cnt = 0;
        const int limit = 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[j * i] = true;
                    }
                }
            }
        }

        return cnt;
    }
};