204. Count Primes

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.

求n以内的素数

C++（31ms）：

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