https://codeforces.com/contest/1139/problem/C

題解：并查集+快速冪

/* *@Author: STZG *@Language: C++ */ #include <bits/stdc++.h> #include<iostream> #include<algorithm> #include<cstdlib> #include<cstring> #include<cstdio> #include<string> #include<vector> #include<bitset> #include<queue> #include<deque> #include<stack> #include<cmath> #include<list> #include<map> #include<set> //#define DEBUG #define RI register int using namespace std; typedef long long ll; //typedef __int128 lll; const int N=200000+10; const int M=100000+10; const int MOD=1e9+7; const double PI = acos(-1.0); const double EXP = 1E-8; const int INF = 0x3f3f3f3f; int t,n,m,k,v,u; ll ans,cnt,flag,temp,sum[N]; int a[N]; char str; int pre[N]; ll powl(ll a, ll n, ll p) //快速冪 a^n % p {ll ans = 1;while(n){if(n & 1) ans = ans * a % p;a = a * a % p;n >>= 1;}return ans; } int find(int x){if(x==pre[x]){return x;}return pre[x]=find(pre[x]); } void add(int x,int y){int tx=find(x);int ty=find(y);if(tx!=ty){pre[tx]=ty;sum[ty]+=sum[tx];} } int main() { #ifdef DEBUGfreopen("input.in", "r", stdin);//freopen("output.out", "w", stdout); #endif//ios::sync_with_stdio(false);//cin.tie(0);//cout.tie(0);//scanf("%d",&t);//while(t--){scanf("%d%d",&n,&k);for(int i=1;i<=n;i++){pre[i]=i;sum[i]=1;}for(int i=1;i<n;i++){scanf("%d%d%d",&u,&v,&t);flag|=t;if(!t)add(u,v);}if(!flag){cout<<0<<endl;return 0;}temp=powl(n,k,MOD);//cout<<temp<<endl;for(int i=1;i<=n;i++){if(pre[i]==i){ans=(ans+powl(sum[i],k,MOD))%MOD;}}ans=(temp-ans+MOD)%MOD;cout<<ans<<endl;//}#ifdef DEBUGprintf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC); #endif//cout << "Hello world!" << endl;return 0; }

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