BZOJ 4517: [Sdoi2016]排列计数 错排+逆元

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 1e6+20, M = 1e6+10, mod = 1e9+7, inf = 1e9+1000;
typedef long long ll;

ll f[N],fac[N];
ll inv(ll x,ll mo)
{
    ll y=mo-2,ans=1;
    while (y)
    {
        if (y&1) ans=ans*x%mo;
        x=x*x%mo;
        y>>=1;
    }
    return ans;
}
void init() {
    fac[0] = 1;
    for(int i=1;i<=M;i++) {
        fac[i] = (fac[i-1]*i)%mod;
    }
    f[0] = 1;
    f[1] = 0;
    f[2] = 1;
    for(ll i=3;i<=M;i++) {
        f[i] = (f[i-1]+f[i-2])%mod*(i-1ll)%mod;
    }
}
int main() {
    init();
    int T;
    scanf("%d",&T);
    while(T--) {
        int n,m;
        scanf("%d%d",&n,&m);
        printf("%lld\n",(f[n-m]*fac[n]%mod*inv(fac[n-m],mod)%mod*inv(fac[m],mod)%mod)%mod);
    }
    return 0;
}