要多简单有多简单。然而不知道为啥在luogu上过不掉。
#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
int read()
{
int x=,f=;char c=getchar();
while (c<''||c>'') {if (c=='-') f=-;c=getchar();}
while (c>=''&&c<='') x=(x<<)+(x<<)+(c^),c=getchar();
return x*f;
}
#define N 1000010
int n,p[N],size[N],t=;
long long ans=;
struct data{int to,nxt,len;
}edge[N<<];
void addedge(int x,int y,int z){t++;edge[t].to=y,edge[t].nxt=p[x],edge[t].len=z,p[x]=t;}
void dfs(int k,int from)
{
size[k]=;
for (int i=p[k];i;i=edge[i].nxt)
if (edge[i].to!=from)
{
dfs(edge[i].to,k);
size[k]+=size[edge[i].to];
ans+=1ll*edge[i].len*abs(n-(size[edge[i].to]<<));
}
}
int main()
{
#ifndef ONLINE_JUDGE
freopen("bzoj2435.in","r",stdin);
freopen("bzoj2435.out","w",stdout);
const char LL[]="%I64d\n";
#else
const char LL[]="%lld\n";
#endif
n=read();
for (int i=;i<n;i++)
{
int x=read(),y=read(),z=read();
addedge(x,y,z),addedge(y,x,z);
}
dfs(,);
cout<<ans;
return ;
}