题解

前缀和+埃式筛法

#include <bits/stdc++.h>
using namespace std;
const int N = 1000001;
bool a[N];
int pre[N];
int main()
{
    int n,m;
    cin >> n >> m;
    for (int i=0;i<=m;i++)a[i] = 1;
    a[1] = 0;
    a[0] = 0;
    for (int i=2;i<=m;i++)
    {
        if (a[i] == 1)
        {
            for (int j=i+i;j<=m;j+=i)
            {
                a[j] = 0;
            }
        }
    }
    pre[0] = 0;
    pre[1] = 0;
    for (int i=2;i<=m+1;i++)
    {
        pre[i] = pre[i-1]+a[i];
    }
    while (n--)
    {
        int p,q;
        cin >> p >> q;
        cout << pre[q]-pre[p-1] << endl;
    }
    return 0;
}