http://www.lydsy.com/JudgeOnline/problem.php?id=4517
组合数+错排公式
#include<cstdio> #include<iostream> using namespace std; #define N 1000001 const int mod=1e9+7; long long fac[N],inv[N],f[N]; void read(int &x) { x=0; char c=getchar(); while(!isdigit(c)) c=getchar(); while(isdigit(c)) { x=x*10+c-'0'; c=getchar(); } } int Pow(long long a,int b) { long long res=1; for(;b;a=a*a%mod,b>>=1) if(b&1) res=res*a%mod; return res; } int main() { freopen("permutation.in","r",stdin); freopen("permutation.out","w",stdout); fac[0]=fac[1]=1; for(int i=1;i<N;++i) fac[i]=fac[i-1]*i%mod; inv[0]=inv[1]=1; for(int i=1;i<N;++i) inv[i]=Pow(fac[i],mod-2); f[0]=f[2]=1; for(int i=3;i<N;++i) f[i]=(i-1)*(f[i-1]+f[i-2])%mod; int T,n,m; read(T); while(T--) { read(n); read(m); printf("%d\n",int(f[n-m]*fac[n]%mod*inv[m]%mod*inv[n-m]%mod)); } fclose(stdin); fclose(stdout); return 0; }