HDOJ2049(错排+组合)

//不是很懂为什么是以M进行错排

#include <iostream> 

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

using namespace std;

__int64 a[25];

void cacu() //错排公式
{
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, c;
cin >> b >> c;
__int64 sum = cacu2(b) / (cacu2(c)*cacu2(b - c));  //组合公式
cout <<a[c]*sum << endl;
}
}