```
class Solution {
public:
int countPrimes(int n) {
if(n<=2) return 0;
if(n==3) return 1;
if(n<=5) return 2;
vector<bool> excluded(n, false);
int test = 1;
int count = 0;
while( test < n-2){
if(test<=2) test++;
else test+=2;
if(excluded[test]) continue;
int m = 2;
while( m*test < n){
excluded[ m * test] = true;
m++;
}
count++;
}
return count;
}
};
```

My third post ;)