204. Count Primes

class Solution {
public:
int countPrimes(int n) {
vector<bool> isprime(n, true);
isprime[0] = false;
isprime[1] = false;
for (int i = 2; i < sqrt(n); i++) {
if (isprime[i]) {
for (int j = i*i; j < n; j += i) {
isprime[j] = false;
}
}
}
return count(isprime.begin(), isprime.end(), true);
}
};