count-primes/submissions/

class Solution {
public:
    int countPrimes(int n) {

        vector <bool> prime (n+1, true);
        int res = 0;

        if (n<2)
            return res;

        prime[0] = false;
        prime[1] = false;

        for (int i = 2 ; i*i <= n ; i++)
        {
            if (prime[i])
            {
                for (int m = i*i ; m <= n ; m += i)
                {
                    prime[m] = false;
                }
            }
        }

       for (int i = 2 ; i < n ; i ++)
            if (prime[i])
                res++;
        return res;

    }
};