Count Primes

 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 array
16         int result = 0;
17         for (int i = 2; i < n; i++) {
18             if (primes[i]) result++;
19         }
20         return result;
21     }
22 };