BZOJ：2190: [SDOI2008]仪仗队

题解：欧拉函数

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;

int n;
int ans=;

int phi[];
int vis[]={};
int prime[],cntprime=;
int Lineshake(){
    vis[]=;phi[]=;
    for(int i=;i<=n;++i){
        if(!vis[i]){
            prime[++cntprime]=i;
            phi[i]=i-;
        }
        for(int j=;(j<=cntprime)&&(i*prime[j]<=n);++j){
            vis[i*prime[j]]=;
            if(i%prime[j]==){
                phi[i*prime[j]]=phi[i]*prime[j];
                break;
            }
            phi[i*prime[j]]=phi[i]*(prime[j]-);
        }
    }
}

int main(){
    scanf("%d",&n);
    Lineshake();
    for(int i=;i<=n-;++i)ans=ans+phi[i]*;
    printf("%d\n",ans+);
    return ;
}