Count Primes

题目描述：

Count the number of prime numbers less than a non-negative number, n.

解题思路：使用素数筛选法

class Solution {
public:
int countPrimes(int n) {
vector<bool> prime(n, true);
prime[0] = false, prime[1] = false;
for (int i = 0; i < sqrt(n); ++i) {
if (prime[i]) {
for (int j = i*i; j < n; j += i) {
prime[j] = false;
}
}
}
return count(prime.begin(), prime.end(), true);
}
};