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 = 22;
int64 a[ maxn ];

void init(){
    a[ 0 ] = a[ 1 ] = 0;
    a[ 2 ] = 1;
    for( int i=3;i<=20;i++ ){
        a[ i ] = ( i-1 )*(a[ i-2 ] + a[ i-1 ]);
    }
    return ;
}

int main(){
    init();
    int n;
    while( scanf("%d",&n)==1 ){
        printf("%I64d
",a[ n ]);
    }
    return 0;
}