BZOJ4517: [Sdoi2016]排列计数(组合数+错位排列)

#include<cstdio>
#define int long long 
using namespace std;
const int mod=1e9+7;
const int MAXN=1e6+10;
inline int read()
{
    char c=getchar();int x=0,f=1;
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
    return x*f;
}
int fac[MAXN],D[MAXN],inv[MAXN];
void Pre()
{
    fac[0]=fac[1]=inv[0]=inv[1]=D[0]=D[2]=1;
    for(int i=2;i<=1000001;i++) fac[i]=(i*fac[i-1])%mod;
    for(int i=2;i<=1000001;i++) inv[i]=(mod-mod/i)*inv[mod%i]%mod;
    for(int i=2;i<=1000001;i++) inv[i]=(inv[i]*inv[i-1])%mod;
    for(int i=3;i<=1000001;i++) D[i]=((i-1)*(D[i-1]+D[i-2]))%mod;
}
int Query(int N,int M)
{
    return fac[N] %mod * inv[ N-M  ] %mod * inv[ M ] %mod * D[N-M] %mod;
}
main()
{
    Pre();
    int T=read();
    while(T--)
    {
        int N=read(),M=read();
        printf("%lld\n",Query(N,M)%mod);
    }
    return 0;
}