筛法求欧拉函数（poj2478

求1～n的欧拉函数的值

#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;
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');
}

int main(){
    a[1]=1;
    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>>n&&n){
        o(a[n]-1);p('\n');
    }
    return 0;
}