CF961G Partitions 第二类斯特林数

这个十分巧妙啊~

#include <bits/stdc++.h>  
#define N  200005   
#define LL long long  
#define mod 1000000007  
#define setIO(s) freopen(s".in","r",stdin)    
using namespace std; 
inline int qpow(int x,int y) 
{
    int tmp=1;  
    for(;y;y>>=1,x=1ll*x*x%mod)    if(y&1)    tmp=1ll*tmp*x%mod;   
    return tmp;   
}
inline int INV(int x) { return qpow(x,mod-2);  }   
int n,K;      
int val[N],inv[N],fac[N];      
inline void Initialize() 
{
    int i,j;   
    fac[0]=inv[0]=1ll;   
    for(i=1;i<N;++i)    fac[i]=1ll*fac[i-1]*i%mod,  inv[i]=INV(fac[i]);         
}
inline int S(int _n,int _m) 
{ 
    int i,j,ans=0;   
    for(i=0;i<=_m;++i)   
    {
        int a=inv[i];   
        if(i&1)     a=mod-a;       
        ans=(ans+1ll*a*inv[_m-i]%mod*qpow(_m-i,_n)%mod)%mod;     
    }
    return ans;   
}
int main() 
{   
    // setIO("input");  
    int i,j,sum=0;  
    scanf("%d%d",&n,&K);   
    for(i=1;i<=n;++i)    scanf("%d",&val[i]),sum=(sum+val[i])%mod;   
    Initialize();                       
    printf("%d\n",1ll*sum*(S(n,K)+1ll*(n-1)*S(n-1,K)%mod)%mod);           
    return 0; 
}