1002-Add or Multiply 1

第二类斯特林数，预处理

//#pragma GCC optimize(3,"Ofast","inline")
#include <bits/stdc++.h>
#define ll long long  
using namespace std;
const int mod = 1e9+7, M = 1000002;
int fa[1010];

ll f[M],s2[3030][3030];

inline int read() {
    int x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}

inline string readstring() {
    string str;
    char s=getchar();
    while(s==' '||s=='\n'||s=='\r'){s=getchar();}
    while(s!=' '&&s!='\n'&&s!='\r'){str+=s;s=getchar();}
    return str;
}

int Find(int x) {
    if(fa[x] == x) return x;
    return fa[x] = Find(fa[x]);
}

void init() 
{
	f[0] = 1;
	for(int i=1; i<1000002; i++)  {
		f[i] = f[i-1] * i % mod;
	}
	s2[1][1] = 1;
	for(int i=2; i<3005; i++) {
		for(int j=1; j<3005; j++) {
			s2[i][j] = (s2[i-1][j-1] + j * s2[i-1][j]) % mod;
		}
	}
}

int main()
{
    init(); 
    int pp = read();
    while(pp--)
    {
    	int n,k;
    	ll z = 0;
		scanf("%lld %lld",&n,&k);
    	for(int j=0; j<=min(n,k); j++)
    	{
    		z = (z + f[j]*f[j]%mod * s2[n+1][j+1]%mod * s2[k+1][j+1]%mod) % mod;
			z %= mod;
			z += mod;
			z %= mod;
		}
		printf("%lld\n",z);
	}
    return 0;
}