题目：

题解：

int countPrimes(int n) {if (n < 2) {return 0;}int isPrime[n];int primes[n], primesSize = 0;memset(isPrime, 0, sizeof(isPrime));for (int i = 2; i < n; ++i) {if (!isPrime[i]) {primes[primesSize++] = i;}for (int j = 0; j < primesSize && i * primes[j] < n; ++j) {isPrime[i * primes[j]] = 1;if (i % primes[j] == 0) {break;}}}return primesSize;
}