P2158 [SDOI2008]仪仗队 线性筛（欧拉函数和素数表）

上三角行恰好是[1,n-1]的欧拉函数

http://www.luogu.org/problem/show?pid=2158#sub

//#pragma comment(linker, "/STACK:167772160")
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <iostream>
#include <queue>
#include <stack>
#include <cmath>
#include <set>
#include <utility>
#include <algorithm>
#include <vector>
#include <map>
// #include<malloc.h>
using namespace std;
#define clc(a,b) memset(a,b,sizeof(a))
#define LL long long
void fre() {
    freopen("in.txt","r",stdin);
}
const int inf = 0x3f3f3f3f;
#define eps 1e-8
// const double pi = acos(-1);
const LL mod = 1e9+7;
inline int r(){
    int x=0,f=1;char ch=getchar();
    while(ch>'9'||ch<'0'){if(ch=='-') f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
const int maxn=1e7;
const int N=40000;
bool vis[maxn];
int phi[maxn];
int prime[maxn];
int tot;
void p_pri(){
   // clc(vis,0);
   phi[1]=1;
   tot=0;
   int i,j;
   for(i=2;i<=N;i++){
      if(!vis[i]){
          prime[tot++]=i;
          phi[i]=i-1;
      }
      for(j=0;j<tot;j++){
         if(i*prime[j]>N) break;
         vis[i*prime[j]]=true;
         if(i%prime[j]==0){
         phi[i*prime[j]]=phi[i]*prime[j];
         break;
         }
         else{
            phi[i*prime[j]]=phi[i]*(prime[j]-1);
         }   
      }
   }
}

int main(){
    int n;
    LL ans;
    ans=0;
    scanf("%d",&n);
    p_pri();
    for(int i=1;i<n;i++){
        ans+=phi[i];
    }
    cout<<ans*2+1<<endl;
    return 0;
}