204.cpp

class Solution {
public:
    int countPrimes(int n) {
        if (n == 1 || n == 0) return 0;
        vector<bool> passed(n, true);
        passed[0] = false, passed[1] = false;
        for(int i = 0; i < sqrt(n); i++) {
            if (passed[i]) {
                for(int j = i * i; j < n; j += i) {
                    passed[j] = false;
                }
            }
        }
        return count(passed.begin(), passed.end(), true);
    }
};