lightoj 1005 组合数学

题目链接：http://lightoj.com/volume_showproblem.php?problem=1005

#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <queue>
#include <vector>
using namespace std;

const int maxe = 50000;
const int maxn = 50;
const int INF  = 0x3f3f3f;

float dp[maxn];
long long C[maxn][maxn];
long long A[maxn][maxn];


int main()
{
    //freopen("E:\acm\input.txt","r",stdin);
    int T;
    cin>>T;
    for(int i=1;i<=30;i++)  C[i][0] = 1;
    A[1][1] = 1;
    for(int i=2;i<=30;i++)  A[i][i] = A[i-1][i-1]*i;
    for(int n=1;n<=30;n++)
        for(int k=1;k<=n;k++){
            C[n][k] = C[n][k-1] * (n-k+1)/k; 
            A[n][k] = C[n][k]*A[k][k];
        }

    for(int t=1;t<=T;t++){
        int n,k;
        cin>>n>>k;
        printf("Case %d: ",t);
        dp[0] = 1;
        if(k > n )     printf("0
");
        else if(k == 0 ) printf("1
");
        else{
            cout<<A[n][k]*C[n][k]<<endl;
        }
    }
}