TopCoder SRM 477 CarelessSecretary 夜

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;

    }
};