bzoj4517: [Sdoi2016]排列计数

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
using namespace std;
typedef long long LL;
const LL mod=1e9+7;

LL f[1100000];
void initf()
{
    f[0]=1;f[1]=0;
    for(int i=2;i<=1000000;i++)f[i]=(f[i-1]+f[i-2])*(i-1)%mod;
}

LL quick_pow(LL A,LL p)
{
    LL ret=1;
    while(p!=0)
    {
        if(p%2==1)ret=(ret*A)%mod;
        A=(A*A)%mod;p/=2;
    }
    return ret;
}
LL ml[1100000],inv[1100000];
void initforC()
{
    ml[0]=1;inv[0]=1;
    for(int i=1;i<=1000000;i++)
        ml[i]=(ml[i-1]*i)%mod, inv[i]=quick_pow(ml[i],mod-2);
}

//-----------init--------------

LL getC(LL n,LL m)
{
    return ((ml[n]*inv[n-m]%mod)*inv[m])%mod;
}

int main()
{
    initf();
    initforC();
    
    int T;
    scanf("%d",&T);
    while(T--)
    {
        int n,m;
        scanf("%d%d",&n,&m);
        printf("%lld\n",(getC(n,m)*f[n-m])%mod);
    }
    return 0;
}