HDU1465+递推

经典的信封装信问题

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

 1 #include<stdio.h>

 2 #include<string.h>

 3 #include<stdlib.h>

 4 typedef long long int64;

 5 const int maxn = 22;

 6 int64 a[ maxn ];

 7 

 8 void init(){

 9     a[ 0 ] = a[ 1 ] = 0;

10     a[ 2 ] = 1;

11     for( int i=3;i<=20;i++ ){

12         a[ i ] = ( i-1 )*(a[ i-2 ] + a[ i-1 ]);

13     }

14     return ;

15 }

16 

17 int main(){

18     init();

19     int n;

20     while( scanf("%d",&n)==1 ){

21         printf("%I64d\n",a[ n ]);

22     }

23     return 0;

24 }

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