Rooks-LightOj1005（规律）

A rook is a piece used in the game of chess which is played on a board of square grids. A rook can only move vertically or horizontally from its current position and two rooks attack each other if one is on the path of the other. In the following figure, the dark squares represent the reachable locations for rook R₁ from its current position. The figure also shows that the rook R₁ and R₂ are in attacking positions where R₁ and R₃ are not. R₂ and R₃ are also in non-attacking positions.

Now, given two numbers n and k, your job is to determine the number of ways one can put k rooks on an n x n chessboard so that  no two of them are in attacking positions.

Input

Input starts with an integer T (≤ 350), denoting the number of test cases.

Each case contains two integers n (1 ≤ n ≤ 30) and k (0 ≤ k ≤ n²).

Output

For each case, print the case number and total number of ways one can put the given number of rooks on a chessboard of the given size so that no two of them are in attacking positions. You may safely assume that this number will be less than 10¹⁷.

Sample Input	Output for Sample Input
8 1 1 2 1 3 1 4 1 4 2 4 3 4 4 4 5	Case 1: 1 Case 2: 4 Case 3: 9 Case 4: 16 Case 5: 72 Case 6: 96 Case 7: 24 Case 8: 0

自我感觉这题大水，30 15这个数据都过不了   然后就对了

但是我感觉人家写的很好，我当时只想到怎么算出来的   并不知道怎么写的

其实就是规律C（m,n）*A(m,n)

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<map>
#include<vector>
#include<stack>
#include<queue>
using namespace std;
const int INF=0x3f3f3f3f;
int main()
{
    long long T,n,m,sum;
    int t=1;
    scanf("%lld",&T);
    while(T--)
    {
        sum=1;
        scanf("%lld %lld",&n,&m);
        for(int i=n;i>(n-m);i--)
        {
            sum*=i;
        }
        for(int i=1;i<=m;i++)
        {
            sum/=i;
        }
        for(int i=n;i>(n-m);i--)
        {
            sum*=i;
        }
        printf("Case %d: %lld
",t++,sum);
    }
    return 0;
}