LeetCode之Count Primes

/*根據提示求解。*/
class Solution {
public:
    int countPrimes(int n) {
        bool mark[n];
        memset(mark, 1, sizeof(mark));
        int sqrt_n = sqrt(n);
        for(int i = 2; i <= sqrt_n; ++i){
            if(mark[i]){
                for(int j = i*i; j < n; j += i){
                mark[j] = false;
              }
            }
        }
        int count = 0; 
        for(int i = 2; i < n; ++i){
            if(mark[i]) ++count;
        }
        return count;
    }
};