【codevs1453 統計素數個數2】E氏篩法

#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);
}