[noi2011]道路修建 树形dp

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<string>
#include<iomanip>
#include<cmath>
using namespace std;
#define LL long long
const int maxn=1003000;
int n;
struct node{
int y,next,v;
}e[maxn<<1];
int linkk[maxn],len=0,siz[maxn],q[maxn],tail=0,fa[maxn],t[maxn],top=0,ru[maxn],w[maxn];
LL ans=0;
void insert(int x,int y,int v){
e[++len].y=y;
e[len].next=linkk[x];
e[len].v=v;
linkk[x]=len;
}
void init(){
scanf("%d",&n);
int x,y,v;
for(int i=1;i<n;i++){
scanf("%d%d%d",&x,&y,&v);
insert(x,y,v);insert(y,x,v);
}
}
void bfs(){
q[++tail]=1;int x;
for(int head=1;head<=tail;head++){
x=q[head];
for(int i=linkk[x];i;i=e[i].next){
if(e[i].y==fa[x])continue;
q[++tail]=e[i].y;
fa[e[i].y]=x;
ru[x]++;
w[e[i].y]=e[i].v;
}
if(!ru[x])t[++top]=x;
}
for(int head=1;head<=top;head++){
x=t[head];siz[x]++;
siz[fa[x]]+=siz[x];
ru[fa[x]]--;
if(!ru[fa[x]])t[++top]=fa[x];
ans+=(LL)abs(n-2*siz[x])*(LL)w[x];
}
}
void work(){
bfs();
cout<<ans<<endl;
}
int main(){
freopen("1.in","r",stdin);
freopen("1.out","w",stdout);
init();
work();
}