Cnm%(个人模版)

 1 #include<stdio.h>
 2 #include<string.h>
 3 #include<vector>
 4 using namespace std;
 5 #define LL __int64
 6 #define MOD 1000000007ll
 7 const LL mod = 1000000007;
 8 const LL N = 100000+5;
 9 const LL M=1e5+3;
10 vector<LL >mp[100500];
11 LL ans;
12 LL n,k;
13 LL vis[100500];
14 LL fac[100005];            //阶乘
15 LL inv_of_fac[100005];        //阶乘的逆元
16 LL qpow(LL x,LL n)
17 {
18     LL ret=1;
19     for(; n; n>>=1)
20     {
21         if(n&1) ret=ret*x%mod;
22         x=x*x%mod;
23     }
24     return ret;
25 }
26 void init()
27 {
28     fac[1]=1;
29     for(int i=2; i<=M; i++)
30         fac[i]=fac[i-1]*i%mod;
31     inv_of_fac[M]=qpow(fac[M],mod-2);
32     for(int i=M-1; i>=0; i--)
33         inv_of_fac[i]=inv_of_fac[i+1]*(i+1)%mod;
34 }
35 LL C(LL a,LL b)
36 {
37     if(b>a) return 0;
38     if(b==0) return 1;
39     return fac[a]*inv_of_fac[b]%mod*inv_of_fac[a-b]%mod;
40 }
41 //(C(k,n)-C(k,cont)-C(k,n-cont)+MOD)%MOD;
42 LL Dfs(int u)
43 {
44     vis[u]=1;
45     LL cont=1;
46     for(LL int i=0;i<mp[u].size();i++)
47     {
48         LL v=mp[u][i];
49         if(vis[v]==0)
50         {
51             LL tmp=Dfs(v);
52             ans=(ans+(C(n,k)%MOD-C(tmp,k)%MOD-C(n-tmp,k)%MOD)%MOD+MOD)%MOD;
53             cont+=tmp;
54         }
55     }
56     return cont;
57 }
58 int main()
59 {
60     while(~scanf("%I64d%I64d",&n,&k))
61     {
62         init();
63         memset(vis,0,sizeof(vis));
64         for(LL i=1;i<=n;i++)mp[i].clear();
65         for(LL i=1;i<=n-1;i++)
66         {
67             LL x,y;
68             scanf("%I64d%I64d",&x,&y);
69             mp[x].push_back(y);
70             mp[y].push_back(x);
71         }
72         ans=0;
73         Dfs(1);
74         printf("%I64d\n",(ans+MOD)%MOD);
75     }
76     //prLLf("%I64d\n",C(5,2));
77 }