CodeForces - 385C Bear and Prime Numbers——素数筛法

在筛素数时计算每个数可以对应多少个题目中给定的数ai， 满足这个数是ai的因子， 然后预处理前缀和，O（1）完成查询

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int maxn = 1e6;
const int maxm = 1e7 + 5;
int n, m, a[maxn], l, r;
bool isprime[maxm];
int vis[maxm], cnt, prime[maxm], num[maxm];
long long sum[maxm];
int maxv;

void init() {
    for (int i = 0; i <= maxv; i++) isprime[i] = true;
    for (int i = 2; i <= maxv; i++) {
        if (!isprime[i]) continue;
        if (vis[i]) num[i]+=vis[i];
        prime[++cnt] = i;
        for (int j = i * 2; j <= maxv; j += i) {
            if (vis[j]) num[i] += vis[j];
            isprime[j] = false;
        }
    }
}

int main() {
    maxv = 0;
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        scanf("%d", &a[i]);
        vis[a[i]]++;
        maxv = max(maxv, a[i]);
    }
    init();
    for (int i = 1; i <= maxv; i++) sum[i] = sum[i-1] + num[i];
    scanf("%d", &m);
    while (m--) {
        scanf("%d %d", &l, &r);
        if (l > maxv) { printf("0\n"); continue; }
        if (r > maxv) r = maxv;
        printf("%lld\n", sum[r] - sum[l - 1]);
    }
    return 0;
}