TopCoder SRM 477 CarelessSecretary

组合+错排

代码：

#include<iostream>

#include<cstring>

#include<cstdio>

#include<string>

#include<set>

#include<map>

#include<cmath>

#include<algorithm>

#include<vector>

#include<cmath>

#include<queue>

#include<stack>



//#define ull unsigned long long

#define ll long long



using namespace std;



const int INF=0x3f3f3f3f;

const int MOD=1000000007;

const ll LMOD=1000000007;

const double eps=1e-6;

const int N=1050;

ll f[N];

ll c[N][N];

class CarelessSecretary

{

    public:

    int howMany(int n,int k)

    {

        f[1]=0;f[2]=1;

        for(int i=3;i<N;++i)

        f[i]=((i-1)*(f[i-1]+f[i-2]))%LMOD;

        memset(c,0,sizeof(c));

        for(int i=0;i<N;++i)

        for(int j=0;j<=i;++j)

        {

            if(i==j||j==0)

            c[i][j]=1;

            else

            c[i][j]=(c[i-1][j]+c[i-1][j-1])%LMOD;

        }

        ll sum=0;

        int w=n-k;

        for(int i=0;i<=w;++i)

        {

            sum=(sum+c[w][i]*f[n-i])%LMOD;

            //cout<<c[w][i]<<" "<<f[n-i]<<endl;

            //cout<<sum<<endl;

        }

        return (int)sum;



    }

};