线性筛

int vis[N];
int prime[N];
int Prime() {
    int index = 0;
    memset(vis, 0, sizeof(vis));
    for (int i = 2; i < N; i++)
    {
        if (vis[i] == 0) {
            prime[index++] = i;
        }
        for (int j = 0; j < index && prime[j] * i < N; j++)
        {
            vis[i * prime[j]] = 1;
            if (i % prime[j] == 0) {
                break;
            }
        }
    }
    return index;
}