AM 1324. Count Primes

Code(Language:C++)

class Solution {
public:
    /**
     * @param n: a integer
     * @return: return a integer
     */
    int countPrimes(int n) {
        // write your code here
        if(n <= 2){
            return 0; 
        }
        std::vector<bool> notPrime(n - 1, false);
        int res = 0;  
        for(int i = 2; i < n; i++){
            if(notPrime[i] == false){
                res++;
            }
            for(int j = 2; i * j < n; j++){
                notPrime[i * j] = true; 
            }
        }
        return res; 
    }
};