HDOJ_1465_不容易系列之一

AC代码：

#include <iostream>
#include <cstdio>
using namespace std;
int main(void)
{
    freopen("in.txt","r",stdin);
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        long long int sum , sum1 = 0, sum2 = 1;
        if(n==1)
            sum=sum1;
        else if(n==2)
            sum=sum2;
        for(int i=3;i<=n;i++)
        {
            sum=(i-1)*sum1+(i-1)*sum2;
            sum1 = sum2;
            sum2 = sum;
        }
        printf("%lld\n",sum);    
    }
    
    
    fclose(stdin);
    system("pause");
    return 0;
}