点分治裸题
#include <iostream>
#include <cstdio>
using namespace std;
int n, uu, vv, ww, ans, cnt, hea[20005], rot, gra[20005], sze, qwq[5];
int siz[20005], gcd;
bool vis[20005];
struct Edge{
int too, nxt, val;
}edge[40005];
void add_edge(int fro, int too, int val){
edge[++cnt].nxt = hea[fro];
edge[cnt].too = too;
edge[cnt].val = val;
hea[fro] = cnt;
}
void getRoot(int x, int f){
siz[x] = 1;
gra[x] = 0;
for(int i=hea[x]; i; i=edge[i].nxt){
int t=edge[i].too;
if(t==f || vis[t]) continue;
getRoot(t, x);
siz[x] += siz[t];
gra[x] = max(gra[x], siz[t]);
}
gra[x] = max(gra[x], sze-siz[x]);
if(gra[rot]>gra[x]) rot = x;
}
void getDeep(int x, int fa, int dis){
qwq[dis]++;
for(int i=hea[x]; i; i=edge[i].nxt){
int t=edge[i].too;
if(!vis[t] && t!=fa)
getDeep(t, x, (dis+edge[i].val)%3);
}
}
int calc(int x, int fa, int dis){
qwq[0] = qwq[1] = qwq[2] = qwq[3] = 0;
getDeep(x, fa, dis%3);
return qwq[0]*qwq[0] + 2 * qwq[1]*qwq[2];
}
void work(int x){
vis[x] = true;
ans += calc(x, x, 0);
for(int i=hea[x]; i; i=edge[i].nxt){
int t=edge[i].too;
if(vis[t]) continue;
ans -= calc(t, x, edge[i].val);
rot = 0;
sze = siz[t];
getRoot(t, 0);
work(rot);
}
}
int getGcd(int x, int y){
return !y?x:getGcd(y, x%y);
}
int main(){
cin>>n;
for(int i=1; i<n; i++){
scanf("%d %d %d", &uu, &vv, &ww);
add_edge(uu, vv, ww);
add_edge(vv, uu, ww);
}
gra[0] = 0x3f3f3f3f;
rot = 0;
sze = n;
getRoot(1, 0);
work(rot);
gcd = getGcd(ans, n*n);
printf("%d/%d
", ans/gcd, n*n/gcd);
return 0;
}