【codevs1453 统计素数个数2】E氏筛法

#include <cstdio>#include <cstring>#include <cmath>bool isp[5000000 + 5];int main(){    int l, r;    scanf("%d %d", &l, &r);    int m = sqrt(r + 0.5);    memset(isp, true, sizeof(isp));    isp[0] = isp[1] = false;    for (int i = 2; i <= m; i++)    {        if (isp[i] == true)        {            for (int j = i * i; j <= r; j += i)            {                isp[j] = false;            }        }    }    int cnt = 0;    for (int i = l; i <= r; i++)    {        if (isp[i] == true)        {            cnt++;        }    }    printf("%d\n", cnt);}