[第一类斯特林数 组合] HDU 4372 Count the Buildings

一个圆排列对应把最大的放在最前

#include<cstdio>
#include<cstdlib>
#include<algorithm>
using namespace std;
typedef long long ll;

const ll P=(ll)1e9+7;
const int N=2000;

ll c[N+5][N+5],s[N+5][N+5];
ll x,y,n,ans;

inline void Pre()
{
c[0][0]=1; s[0][0]=1;
for (int i=1;i<=N;i++)
{
c[i][0]=1; s[i][0]=0;
for (int j=1;j<=i;j++)
{
c[i][j]=(c[i-1][j]+c[i-1][j-1])%P;
s[i][j]=(s[i-1][j-1]+s[i-1][j]*(i-1)%P)%P;
}
}

}

int main()
{
int T;
freopen("t.in","r",stdin);
freopen("t.out","w",stdout);
Pre();
scanf("%d",&T);
while (T--)
{
scanf("%lld%lld%lld",&n,&x,&y);
ans=c[x+y-2][x-1]*s[n-1][x+y-2]%P;
printf("%lld\n",ans);
}
return 0;
}