HDOJ2048(错位排列)

#include <iostream> 
#include <cstring>
#include <string>
#include <cstdio>
#include <algorithm>
#include <stack>
#include <math.h>

using namespace std;

//(i - 1)*(a[i - 1] + a[i - 2])
__int64 a[25];


void cacu() //每种人数下出现全错排的种类数
{
a[1] = 0;
a[2] = 1;
for (int i = 3; i <= 20; i++) a[i] = (i - 1)*(a[i - 1] + a[i - 2]);
}


__int64 cacu2(int b)
{
__int64 sum = 1;
for (int i = 2; i <= b; i++) sum *= i;
return sum;
}


int main()
{
cacu();
int t;
cin >> t;
for (int i = 1; i <= t; i++)
{
int b;
cin >> b;
printf("%.2lf%%\n", (double)a[b] / cacu2(b)*100);  //f都两个% 
}
}