题目：

题解：

class Solution {
public:
    int countPrimes(int n) {
        vector<int> primes;
        vector<int> isPrime(n, 1);
        for (int i = 2; i < n; ++i) {
            if (isPrime[i]) {
                primes.push_back(i);
            }
            for (int j = 0; j < primes.size() && i * primes[j] < n; ++j) {
                isPrime[i * primes[j]] = 0;
                if (i % primes[j] == 0) {
                    break;
                }
            }
        }
        return primes.size();
    }
};