poj2478-Farey Sequence

http://poj.org/problem?id=2478

素数筛法+欧拉函数

 

#include<iostream>
#include<cstdio>
#include<cstring>

using namespace std; 

#define maxn  1000005 
#define INT __int64 
int Prime[ maxn + 1 ] ;
INT ans[ maxn + 1 ] ;

void prime()
{
	memset( Prime , 0 , sizeof( Prime ) ) ;
	Prime[ 0 ] = Prime[ 1 ] = 1 ;
	for( int i = 2 ; i * i <= maxn ; ++i )
	{
		if( !Prime[ i ] )
		{
			for( int j = i * i ; j <= maxn ; j += i )
				Prime[  j ] = 1 ;
		}
	}
}

void enlerfun()
{
	for( int i = 1 ; i <= maxn ; ++i )
		ans[ i ] = i ;
	for( int i = 2 ; i <= maxn ; ++i )
	{
		if( !Prime[ i ] )
		for( int j = i ; j <= maxn ; j += i )
		{
			ans[ j ] = ans[ j ] / i * ( i - 1 ) ;
		}
	}
}
	
int main()
{
	int n ;
	prime() ;
	enlerfun() ;
	for( int i = 3 ; i <= maxn ; ++i )
		ans[ i ] += ans[ i - 1 ] ;
	while( ~scanf( "%d" , &n ) && n )
	{
		printf( "%I64d\n" , ans[ n ] ) ;
	}
	return 0 ;
}