[LeetCode]Count Primes

Description:

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

参考https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

class Solution {
public:
    int countPrimes(int n) {
        if(n<=2)
            return 0;
        vector<bool> isprime(n,true);
        for(int i=3; i*i<n; i=i+2){
            if(isprime[i]){
                for(int j=i*i; j<n; j+=2*i){ //隔开n个乘积,跳过偶数还有优化
                    isprime[j] = false;
                    
                }
            }
        }
        int count = 1;
        for(int i=3; i<n; i+=2){
            if(isprime[i]) count++;
        }
        return count;
    }
};