204. Count Primes（LeetCode）

Description:

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

class Solution {
public:
    int countPrimes(int n) {
      vector<bool> num(n - 1, true);
        num[0] = false;
        int res = 0, limit = sqrt(n);
        for (int i = 2; i <= limit; ++i) {
            if (num[i - 1]) {
                for (int j = i * i; j < n; j += i) {
                    num[j - 1] = false;
                }
            }
        }
        for (int j = 0; j < n - 1; ++j) {
            if (num[j]) ++res;
        }
        return res;
    }
};