TopCoder SRM 457 TheHexagonsDivOne

容斥 组合排列

代码：

#include <iostream>

#include <cstdio>

#include <map>

#include <algorithm>

#include <cstring>

#include <string>



#define ll long long



using namespace std;



const int N=1005;

const long long LINF=(long long)(1e18);

ll c[N][N];

ll a[N];

void FC(int n)

{

    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-1]+c[i-1][j];

    a[0]=1;

    for(int i=1;i<=n;++i)

    a[i]=a[i-1]*i;

}

class TheHexagonsDivOne

{

    public:

    long long count(int n)

    {

        FC(2*n);

        if(n<=3)

        return 0;

        ll tmp=2*n;

        --n;

        ll sum=0;

        sum+=(c[2*n][6]*a[6]/6);

        sum-=(c[n][1]*c[2*(n-1)][4]*12*a[4]/6);

        sum+=(c[n][2]*c[2*(n-2)][2]*12*6*a[2]/6);

        sum-=(c[n][3]*c[2*(n-3)][0]*12*4*2*a[0]/6);

        return sum*tmp;

    }

};