牛客练习赛1 树

https://ac.nowcoder.com/acm/problem/13611

其实把，把树分成k个连通块有几种分配方法= ====== 从树上取下k-1条边有几种取法，排列组合就行了，，，，，，

#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
const ll mod = 1e9+7;

//(a/c) % p == a * c^(p-2) %  p
ll k_q(ll a,ll b){
	ll res = 1;
	while(b){
		if(b&1){
			res = (res*a)%mod;
		}
		b>>=1;
		a = (a*a)%mod;
	}
	return res%mod;
}
ll list[20000];
ll cns[20000];
ll C(ll n,ll m){
	ll ans = (((list[n]*cns[n-m])%mod)*cns[m])%mod;
	return ans;
}


int main(){
	list[0]=1;
	cns[0]=1;
	for(int i = 1;i<=2000;i++){
		list[i] =  (list[i-1]*i)%mod; 
		cns[i] = k_q(list[i],mod-2)%mod;
	}
	int n,k;
	cin>>n>>k;
	list[0] = 1;
	
	int be,en;
	for(int i=1;i<n;i++){
		cin>>be>>en;
	}
	ll ans = 0;
	for(int i=1;i<=min(n,k);i++){
		ll c = (((C(n-1,i-1)*C(k,i))%mod )*list[i])%mod;
		ans = (ans+c)%mod;
	}
	cout<<ans<<endl;
	return 0;
}