学步园

这题跟hdu 2049 差不多,那个是从N个中选M个,这个是从N个从选1,2,3.......N/2个,错排与组合.....

#include<stdio.h>
long long num[28];
void chart(  )
{
     num[0] = 1;
     num[1] = 0;
     num[2] = 1;
     for( int i = 3; i < 28; ++i )
          num[i] = ( i - 1 )*( num[i-1] + num[i-2] );
 }
long long f( int n,int k  )
{
     double sum = 1;
     for( int i = n,j = 1; j <= k; ++j,--i )
          sum *= i,sum /=j;
     return ( long long )sum;
 }
long long cal( int n )
{
     long long sum = 0;
     for( int i = 0; i <= n / 2; ++i )
          sum += num[i] * f( n,i );
     return sum;
 }
int main( )
{
    chart( );
    int n;
    while( scanf( "%d",&n ),n )
    {
           printf( "%I64d\n",cal( n ) );
           }
    return 0;
}