luogu 1865 数论 线性素数筛法

洛谷 1865 数论 线性素数筛法

#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\n");
continue;
}
printf("%d\n", sum[r] - sum[l-1]);
}
return 0;
}