hdu 2824 线性筛素数求欧拉函数＋前缀和

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#define MAX 3000001

using namespace std;

int p[1000000],pt;
long long phi[MAX];
bool  m[MAX];
int N = 3000001;
void init ()
{
    memset ( m , 0 , sizeof ( m ) );
    pt = 0;
    phi[1] = 1;
    int k;
    for ( int i = 2 ; i < N ; i++ )
    {
        if (!m[i]) p[pt++]=i,phi[i]=i-1,m[i]=1;
        for ( int j=0;j<pt&&(k=i*p[j])<N;j++)
        {
            m[k] = 1;
            if ( i%p[j]==0 )
            {
                phi[k]=phi[i]*p[j];
            }
            else phi[k]=phi[i]*(p[j]-1);
        }
    } 
} 

int main ( )
{
    init ();
    for ( int i = 2 ; i < N ; i++ )
        phi[i] = phi[i]+phi[i-1];
    int left , right;
    while ( ~scanf ( "%d%d" , &left , &right ) )
        printf ( "%I64d\n" , phi[right]-phi[left-1] );
    return 0;
}