204. Count Primes(埃拉托色尼）

Description:

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

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

class Solution {
public:
    int countPrimes(int n) {
        if(n <= 2) return 0;
        vector<int> prime(n+1, -1);
        for(int i=2; i<sqrt(n); ++i){
            if(prime[i] != 0){
                int p = i + i;
                while(p < n){
                    prime[p] = 0;
                    p += i;  //这里是+i,不要写走眼
                }
            }
        }

        int count = 0;
        for(int i=2; iif(prime[i] != 0)
                ++count;
        }

        return count;
    }
};