luogu 1865 数论 线性素数筛法

#洛谷 1865 数论 线性素数筛法

最基本的线性素数筛法，当做复习欧拉筛法了，没有尝试过使用更暴力的筛法。。。

WA了一次，手抖没打

传送门 (https://www.luogu.org/problem/show?pid=1865)

#include <cstdio>
#include <cstring>
#include <algorithm>

const int maxn = 1000000 + 500;

int prime[maxn];
int isp[maxn];
int n, m;
int sum[maxn];
int l, r;
int tot = 0;
int main () {
    scanf("%d %d", &n, &m);
    for (int i = 2; i <= m; i++) isp[i] = 1;
    for (int i = 2; i <= m; i++) {
        if (isp[i]) {
            tot++;
            prime[tot] = i;
        }
        long long k;
        for (int j = 1; j <= tot && (k = 1ll * i * prime[j]) <= m; j++) {
            isp[k] = 0;
            if (!(i % prime[j])) break;
        }
    }
    for (int i = 2; i <= m; i++) sum[i] = sum[i-1] + (isp[i]);
    for (int i = 1; i <= n; i++) {
        scanf("%d %d", &l, &r);
        if (l < 1 || r > m) {
            printf("Crossing the line
");
            continue;
        }
        printf("%d
", sum[r] - sum[l-1]);
    }
    return 0;
}