P2634 [国家集训队]聪聪可可

点分治傻逼题。。。直接DP就行了

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<algorithm>
#define LL long long
using namespace std;
const int maxx = 2e5+6;
int ver[maxx],sz[maxx],Next[maxx],edge[maxx],head[maxx],mx[maxx];
int vis[maxx];
int dis[maxx];
int tot,ans,n,m,root,size;
inline int read() {
    char ch = getchar(); int x = 0, f = 1;
    while(ch < '0' || ch > '9') {
        if(ch == '-') f = -1;
        ch = getchar();
    } while('0' <= ch && ch <= '9') {
        x = x * 10 + ch - '0';
        ch = getchar();
    } return x * f;
}
int gcd(int a,int b){
   return (!b)?a:gcd(b,a%b);
}
void add(int u,int v,int w){
  ver[++tot]=v;edge[tot]=w;Next[tot]=head[u];head[u]=tot;
  ver[++tot]=u;edge[tot]=w;Next[tot]=head[v];head[v]=tot;
}
void getroot(int u,int fa){
    sz[u]=1;
    mx[u]=0;
    for (int i=head[u];i;i=Next[i]){
        int v=ver[i];
        if (v==fa||vis[v])continue;
        getroot(v,u);
        sz[u]+=sz[v];
        mx[u]=max(mx[u],sz[v]);
    }
    mx[u]=max(mx[u],size-mx[u]);
    if(!root||mx[u]<mx[root]){
       root=u;
    }
}
void getdis(int u,int fa,int w){
   dis[w%3]++;
   for (int i=head[u];i;i=Next[i]){
      int v=ver[i];
      if (v==fa || vis[v])continue;
      getdis(v,u,w+edge[i]);
   }
}
LL cal(int u,int w){
   dis[0]=dis[1]=dis[2]=0;
 //  dis[w%3]++;
   getdis(u,0,w);
   return dis[1]*dis[2]*2+dis[0]*dis[0];
}
LL dfs(int u){
    vis[u]=1;
    LL s=cal(u,0);
    ans+=s;
    for (int i=head[u];i;i=Next[i]){
        int v=ver[i];
        if (vis[v])continue;
        ans-=cal(v,edge[i]);
        root=0;
        size=sz[v];
        getroot(v,0);
        dfs(root);
    }
}
int main(){
   int u,v,w;
       n=read();
       for (int i=1;i<n;i++){
        u=read();
        v=read();
        w=read();
        add(u,v,w);
       }
       ans=0;
       size=n;
       root=0;
       getroot(1,0);
       dfs(root);
       int gc=gcd(n*n,ans);
       printf("%d/%d\n",ans/gc,n*n/gc);
   return 0;
}