HDU1465+递推

经典的信封装信问题

f[ n ]  = ( n-1 ) * ( f[ n-1 ]+f[ n-2 ] )

 #include<stdio.h>
 #include<string.h>
 #include<stdlib.h>
 typedef long long int64;
 const int maxn = ;
 int64 a[ maxn ];

 void init(){
     a[  ] = a[  ] = ;
     a[  ] = ;
     for( int i=;i<=;i++ ){
         a[ i ] = ( i- )*(a[ i- ] + a[ i- ]);
     }
     return ;
 }

 int main(){
     init();
     int n;
     while( scanf("%d",&n)== ){
         printf("%I64d\n",a[ n ]);
     }
     return ;
 }