https://cn.vjudge.net/problem/HDU-4372

最高的（n）左右都可见；将左右两边分成f-1和b-1组；每组只有其中最高得可见；每组间可以任意排列（循环排列）；就是第一类斯特林数了；

#include <iostream>
#include <cstring>
#include <queue>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <set>
#include <map>
#include <stack>
//#include <tr1/unordered_map>
//#include <unordered_map>
#include <cmath>
//#include<bits/stdc++.h>
using namespace std;

#define sfi(i) scanf("%d",&i)
#define sfl(i) scanf("%I64d",&i)
#define sfs(i) scanf("%s",(i))
#define pri(i) printf("%d\n",i)
#define prl(i) printf("%I64d\n",i)
#define sff(i) scanf("%lf",&i)
#define ll long long
#define ull unsigned long long
#define uint unsigned int
#define mem(x,y) memset(x,y,sizeof(x))
#define INF 0x3f3f3f3f
#define inf 1e18
#define eps 1e-10
#define PI acos(-1.0)
#define lowbit(x) ((x)&(-x))
#define fl() printf("flag\n")
#define MOD(x) ((x%mod)+mod)%mod
#define endl '\n'
#define pb push_back
#define lson (rt<<1)
#define rson (rt<<1|1)
#define FAST_IO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)

template<typename T>inline void read(T &x)
{
    x=0;
    static int p;p=1;
    static char c;c=getchar();
    while(!isdigit(c)){if(c=='-')p=-1;c=getchar();}
    while(isdigit(c)) {x=(x<<1)+(x<<3)+(c-48);c=getchar();}
    x*=p;
}

//-----------------------------------------------

const int maxn=2e3+9;
const int maxm=3e6+9;
const int mod=1e9+7;

ll s[maxn][maxn],c[maxn][maxn];

int main()
{
    //FAST_IO;
    //freopen("input.txt","r",stdin);

    s[1][0]=0;
    s[1][1]=1;
    c[1][0]=1;
    c[1][1]=1;
    for(int i=2;i<=2000;i++)
    {
        c[i][0]=1;
        for(int j=1;j<=i;j++)
        {
            c[i][j]=(c[i-1][j]+c[i-1][j-1])%mod;
            s[i][j]=(s[i-1][j]*(i-1)%mod+s[i-1][j-1])%mod;
        }
    }



    int T;
    sfi(T);
    //cin>>T;
    while(T--)
    {
        int n,f,b;
        scanf("%d%d%d",&n,&f,&b);
        if(f+b-2>n) printf("0\n");
        else printf("%lld\n",c[f+b-2][f-1]*s[n-1][f+b-2]%mod);
    }

    return 0;
}