1 class Solution { 2 public: 3     int countPrimes(int n) { 4         vector<bool> primes(n, true); 5          6         // label all non-prime numbers 7         for (int i = 2; i < n; i++) { 8             if (!primes[i]) continue; 9             int count = 2;10             while (count * i <= n) {11                 primes[(count++) * i] = false;12             }13         }14         15         // count how many primes in the array16         int result = 0;17         for (int i = 2; i < n; i++) {18             if (primes[i]) result++;19         }20         return result;21     }22 };