欧拉筛板子

vector<int>v;
int vis[N];
int n;
void prime(){
int num = 0;
v.push_back(0);
clears(vis,0);
vis[0] = 1,vis[1] = 1;
for( int i = 2 ; i <= n ; i++ ){
if(!vis[i]) v.push_back(i);
for( int j = 1 ; j <= v.size() ; j++ ){
if(i * v[j] > n) break;
vis[i * v[j]] = 1;
if(i % v[j] == 0) break;
}
}
}