P4071 [SDOI2016]排列计数

#define int long long

int n,m;
const int N=1e6+10,mod=1e9+7;
int D[N],fac[N]; 

inline int ksm(int a,int k,int mod){
    int res=1;
    for(;k;k>>=1){
        if(k&1)
            res=res*a%mod;
        a=a*a%mod;
    }
    return res;
}

#undef int
int main(){
#define int long long
    freopen("test.txt","r",stdin);
    int T;rd(T);
    D[0]=1,D[1]=0;
    rep(i,2,N-10)
        D[i]=(D[i-1]+D[i-2])*(i-1)%mod;
    fac[0]=1;
    rep(i,1,N-10)
        fac[i]=fac[i-1]*i%mod;
    while(T--){
        rd(n),rd(m);
        printf("%lld\n",fac[n]*ksm(fac[m],mod-2,mod)%mod*ksm(fac[n-m],mod-2,mod)%mod*D[n-m]%mod);
    }
    return 0;
}