欧拉工程第72题：Counting fractions

public class P72{
void run(){
int max_n = 1000000;
long[] phi = cal_phi(max_n+1);
long sum=0;
phi[1] = 0;
for(int i =1;i<=max_n;i++)
sum+=phi[i];
System.out.println(sum);
}
//    303963552391
//    194ms
long[] cal_phi(int max_n){
long[] phi = new long[max_n+1];
for(int i=1;i<max_n;i++){
phi[i] += i;
for(int j =2*i;j<max_n;j+=i)
phi[j]-=phi[i];
}
return phi;
}
public static void main(String[] args){
long t0 = System.currentTimeMillis();
new P72().run();
long t1= System.currentTimeMillis();
System.out.println((t1-t0)+"ms");
}
}