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));

 }