UVA - 11327

UVA - 11327
https://vjudge.net/problem/28358/origin
求欧拉函数的前缀和，二分查找到那个位置，再从它开始暴力gcd找

#include <iostream>
#include <cstdio>
#include <queue>
#include <algorithm>
#include <cmath>
#include <cstring>
#define inf 2147483647
#define N 1000010
#define p(a) putchar(a)
#define For(i,a,b) for(long long i=a;i<=b;++i)

using namespace std;
long long n,cnt;
long long a[N];
void in(long long &x){
    long long y=1;char c=getchar();x=0;
    while(c<'0'||c>'9'){if(c=='-')y=-1;c=getchar();}
    while(c<='9'&&c>='0'){ x=(x<<1)+(x<<3)+c-'0';c=getchar();}
    x*=y;
}
void o(long long x){
    if(x<0){p('-');x=-x;}
    if(x>9)o(x/10);
    p(x%10+'0');
}

long long gcd(long long a,long long b){
    return a%b==0?b:gcd(b,a%b);
}

int main(){
    a[1]=2;
    For(i,2,1e6)
        if(!a[i])
            for(long long j=i;j<=1e6;j+=i){
                if(!a[j]) a[j]=j;
                a[j]=a[j]/i*(i-1);
            }   
    For(i,1,1e6)
        a[i]+=a[i-1];
    while(cin>>cnt&&cnt){
        n=lower_bound(a+1,a+1000000+1,cnt)-a;
        cnt-=a[n-1];
        For(i,0,n){
            if(gcd(i,n)==1)
                cnt--;    
            if(!cnt){
                o(i);p('/');o(n);p('\n');
                break;
            }                  
        }
    }
    return 0;
}