TopCoder SRM 457 TheHexagonsDivOne 夜

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;
    }
};