Rank

Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 433 Accepted Submission(s): 207

Problem Description

Recently in Teddy's hometown there is a competition named "Cow Year Blow Cow".N competitors had took part in this competition.The competition was so intense that the rank was changing and changing.
Now the question is:
How many different ways that n competitors can rank in a competition, allowing for the possibility of ties.
as the answer will be very large,you can just output the answer MOD 20090126.
Here are the ways when N = 2:
P1 < P2
P2 < P1
P1 = P2

Input

The first line will contain a T,then T cases followed.
each case only contain one integer N (N <= 100),indicating the number of people.

Output

One integer pey line represent the answer MOD 20090126.

Sample Input

2
2
3

Sample Output

3
13

Author

teddy

Source

百万秦关终属楚

第二类Stirling数 S(p,k)

S(p,k)的一个组合学解释是：将p个物体划分成k个非空的不可辨别的（可以理解为盒子没有编号）集合的方法数。

k!S(p,k)是把p个人分进k间有差别(如：被标有房号）的房间(无空房）的方法数。

S(p,k)的递推公式是：S(p,k)=k*S(p-1,k)+S(p-1,k-1) ,1<= k<=p-1

边界条件：S(p,p)=1 ,p>=0 S(p,0)=0 ,p>=1

递推关系的说明：

考虑第p个物品，p可以单独构成一个非空集合，此时前p-1个物品构成k-1个非空的不可辨别的集合，方法数为S(p-1,k-1)；

也可以前p-1种物品构成k个非空的不可辨别的集合，第p个物品放入任意一个中，这样有k*S(p-1,k)种方法。

为n个队员分配k个名次，k的取值为1到n。可将问题转化为将n个球正好放入k的不同的盒子，即为第二类斯特林数，因为k个盒子不同，存在k！个排列，所以为k！× S2[n][k]，答案为sum(i!*S2[n][i]) | 1<=i<=k

 //2017-08-05

 #include <cstdio>

 #include <cstring>

 #include <iostream>

 #include <algorithm>

 #define ll long long

 using namespace std;

 const int N = ;

 const int MOD = ;

 ll stir2[N][N], ans[N], factorial[N];

 void init(){

     factorial[] = ;

     for(int i = ; i < N; i++)

           factorial[i] = (factorial[i-]*i)%MOD;

     memset(stir2, , sizeof(stir2));

     for(int n = ; n < N; n++){

         stir2[n][] = ;

         stir2[n][n] = ;

         for(int k = ; k < n; k++){

             stir2[n][k] = stir2[n-][k-]+k*stir2[n-][k];

             stir2[n][k] %= MOD;

         }

     }

 }

 int main()

 {

     int T, n;

     init();

     cin>>T;

     while(T--){

         cin>>n;

         ll ans = ;

         for(int i = ; i <= n; i++)

               ans = (ans + factorial[i]*stir2[n][i]) % MOD;

         cout<<ans<<endl;

     }

     return ;

 }

第二类

HDU2643(SummerTrainingDay05-P 第二类斯特林数)