Leetcode 204 Count Primes

Description:

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

统计小于n的素数有多少个。

用筛法进行素数打表，边打表边记录个数。

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