P4071 [SDOI2016]排列计数
错位排列板子题，plus: 组合数取模
const int N=1e6;
#define int long long
const int mod =1e9+7 ;
int n,m,D[N+3] ;
#define ll long long
ll inv[N+3];
int F[N+3] ;
int fpow(int n, int p, int mod){
n %= mod;
int ans = 1, base = n;
while(p){
if(p & 1) ans = ans * base % mod;
base = base * base % mod;
p >>= 1;
}
return ans;
}
void get_inv(ll n,ll p){
inv[N] = fpow(F[N], mod - 2, mod);
for(int i = N - 1; i >= 0; --i) inv[i] = inv[i + 1] * (i + 1) % mod;
}
ll C(ll n,ll m){
return F[n]*inv[m]%mod*inv[n-m]%mod;
}
void sov(){
cout << (C(n,m)*D[n-m])%mod <<endl;
}
signed main() {
F[0]=1;
for(int i=1;i<=1e6;i++) F[i]=F[i-1]*i,F[i]%=mod;
get_inv(1e6,mod);
D[0]=0,D[1]=0,D[2]=1;
for(int i=3;i<=1e6;i++)
D[i] =(i-1)*(D[i-1]+D[i-2]),D[i]%=mod;
int tes;
cin>>tes;
while(tes--){
cin>>n>>m;
if(n==m) cout<<1<<endl; else sov() ;
}
}