hdu4556-Stern-Brocot Tree

 

http://acm.hdu.edu.cn/showproblem.php?pid=4556

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

using namespace std ;

#define maxn 1000005
#define INT __int64
INT ans[ maxn + 1] ;
void enlerfun()
{
	memset( ans , 0 , sizeof( ans ) );
	ans[ 1 ] = 1 ;
	for( int i = 2 ; i <= maxn ; ++i )
	{
		if( !ans[ i ] )
		{
			for( int j = i ; j <= maxn ; j += i )
			{
				if( !ans[ j ] )
					ans[ j ] = j ;
				ans[ j ] = ans[ j ] / i * ( i - 1 ) ;
			}
		}
		ans[ i ] += ans[ i - 1 ] ;
	}
}

int main()
{
	int n ;
	enlerfun() ;
	while( scanf( "%d" , &n ) != EOF  && n )
	{
		printf( "%I64d\n" , 2 * ans[ n ] + 1 ) ;
	}
	return 0 ;
}