P4071 [SDOI2016]排列计数

 

错位排列板子题，plus: 组合数取模

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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() ;     }  }