204 Count Primes

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