HDU4556 SternBrocot Tree Farey数列

　　题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=4556

　　题目是要求Farey数列：分子和分母小于等于n的所有最简分数的个数，n每增加1，由于分数最简，那么分子只能与分母互素，个数即为phi(n)。

//STATUS:C++_AC_109MS_15896KB
#include <functional>
#include <algorithm>
#include <iostream>
//#include <ext/rope>
#include <fstream>
#include <sstream>
#include <iomanip>
#include <numeric>
#include <cstring>
#include <cassert>
#include <cstdio>
#include <string>
#include <vector>
#include <bitset>
#include <queue>
#include <stack>
#include <cmath>
#include <ctime>
#include <list>
#include <set>
#include <map>
using namespace std;
//define
#define pii pair<int,int>
#define mem(a,b) memset(a,b,sizeof(a))
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define PI acos(-1.0)
//typedef
typedef __int64 LL;
typedef unsigned __int64 ULL;
//const
const int N=1000010;
const int INF=0x3f3f3f3f;
const int MOD=256,STA=8000010;
const LL LNF=1LL<<60;
const double EPS=1e-8;
const double OO=1e15;
const int dx[4]={-1,0,1,0};
const int dy[4]={0,1,0,-1};
const int day[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
//Daily Use ...
inline int sign(double x){return (x>EPS)-(x<-EPS);}
template<class T> T gcd(T a,T b){return b?gcd(b,a%b):a;}
template<class T> T lcm(T a,T b){return a/gcd(a,b)*b;}
template<class T> inline T Min(T a,T b){return a<b?a:b;}
template<class T> inline T Max(T a,T b){return a>b?a:b;}
template<class T> inline T Min(T a,T b,T c){return min(min(a, b),c);}
template<class T> inline T Max(T a,T b,T c){return max(max(a, b),c);}
template<class T> inline T Min(T a,T b,T c,T d){return min(min(a, b),min(c,d));}
template<class T> inline T Max(T a,T b,T c,T d){return max(max(a, b),max(c,d));}
//End

LL phi[N],f[N];
int n;

void phitable(int n)
{
    int i,j;
    for(i=0;i<=n;i++)phi[i]=0;
    phi[1]=1;
    for(i=2;i<=n;i++)if(!phi[i]){
        for(j=i;j<=n;j+=i){
            if(!phi[j])phi[j]=j;
            phi[j]=phi[j]/i*(i-1);
        }
    }
}

int main()
{
 //   freopen("in.txt","r",stdin);
    int i,j,m=N-9;
    phitable(m);
    f[1]=2;
    for(i=2;i<=m;i++)
        f[i]=f[i-1]+phi[i];
    for(i=1;i<=m;i++)
        f[i]+=f[i]-1;
    while(~scanf("%d",&n))
    {
        printf("%I64d\n",f[n]);
    }
    return 0;
}