Cnm%(个人模版)

Cnm%：

 #include<stdio.h>
 #include<string.h>
 #include<vector>
 using namespace std;
 #define LL __int64
 #define MOD 1000000007ll
 const LL mod = ;
 const LL N = +;
 const LL M=1e5+;
 vector<LL >mp[];
 LL ans;
 LL n,k;
 LL vis[];
 LL fac[];            //阶乘
 LL inv_of_fac[];        //阶乘的逆元
 LL qpow(LL x,LL n)
 {
     LL ret=;
     for(; n; n>>=)
     {
         if(n&) ret=ret*x%mod;
         x=x*x%mod;
     }
     return ret;
 }
 void init()
 {
     fac[]=;
     for(int i=; i<=M; i++)
         fac[i]=fac[i-]*i%mod;
     inv_of_fac[M]=qpow(fac[M],mod-);
     for(int i=M-; i>=; i--)
         inv_of_fac[i]=inv_of_fac[i+]*(i+)%mod;
 }
 LL C(LL a,LL b)
 {
     if(b>a) return ;
     if(b==) return ;
     return fac[a]*inv_of_fac[b]%mod*inv_of_fac[a-b]%mod;
 }
 //(C(k,n)-C(k,cont)-C(k,n-cont)+MOD)%MOD;
 LL Dfs(int u)
 {
     vis[u]=;
     LL cont=;
     for(LL int i=;i<mp[u].size();i++)
     {
         LL v=mp[u][i];
         if(vis[v]==)
         {
             LL tmp=Dfs(v);
             ans=(ans+(C(n,k)%MOD-C(tmp,k)%MOD-C(n-tmp,k)%MOD)%MOD+MOD)%MOD;
             cont+=tmp;
         }
     }
     return cont;
 }
 int main()
 {
     while(~scanf("%I64d%I64d",&n,&k))
     {
         init();
         memset(vis,,sizeof(vis));
         for(LL i=;i<=n;i++)mp[i].clear();
         for(LL i=;i<=n-;i++)
         {
             LL x,y;
             scanf("%I64d%I64d",&x,&y);
             mp[x].push_back(y);
             mp[y].push_back(x);
         }
         ans=;
         Dfs();
         printf("%I64d\n",(ans+MOD)%MOD);
     }
     //prLLf("%I64d\n",C(5,2));
 }