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+;
 inline int r(){
     int x=,f=;char ch=getchar();
     while(ch>''||ch<''){if(ch=='-') f=-;ch=getchar();}
     while(ch>=''&&ch<=''){x=x*+ch-'';ch=getchar();}
     return x*f;
 }
 const int maxn=1e7;
 const int N=;
 bool vis[maxn];
 int phi[maxn];
 int prime[maxn];
 int tot;
 void p_pri(){
    // clc(vis,0);
    phi[]=;
    tot=;
    int i,j;
    for(i=;i<=N;i++){
       if(!vis[i]){
           prime[tot++]=i;
           phi[i]=i-;
       }
       for(j=;j<tot;j++){
          if(i*prime[j]>N) break;
          vis[i*prime[j]]=true;
          if(i%prime[j]==){
          phi[i*prime[j]]=phi[i]*prime[j];
          break;
          }
          else{
             phi[i*prime[j]]=phi[i]*(prime[j]-);
          }
       }
    }
 }

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