bzoj2435: [Noi2011]道路修建

#include<cstdio>
#include<cstring>
#include<cctype>
#include<algorithm>
using namespace std;
#define rep(i,s,t) for(int i=s;i<=t;i++)
#define dwn(i,s,t) for(int i=s;i>=t;i--)
#define clr(x,c) memset(x,c,sizeof(x))
#define qwq(x) for(edge *o=head[x];o;o=o->next)
#define ll long long
int read(){
    int x=0;char c=getchar();
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) x=x*10+c-‘0‘,c=getchar();
    return x;
}
const int nmax=1e6+5;
const int inf=0x7f7f7f7f;
struct edge{
    int to,dist;edge *next;
};
edge es[nmax<<1],*pt=es,*head[nmax];
void add(int u,int v,int d){
    pt->to=v;pt->dist=d;pt->next=head[u];head[u]=pt++;
    pt->to=u;pt->dist=d;pt->next=head[v];head[v]=pt++;
}
ll ans=0;int n;
int dfs(int x,int fa){
    int cnt=1;
    qwq(x) if(o->to!=fa){
        int tmp=dfs(o->to,x);
        cnt+=tmp;ans+=(ll)abs(n-tmp-tmp)*o->dist;
    }
    return cnt;
}
int main(){
    n=read();int u,v,d;
    rep(i,1,n-1){
        u=read(),v=read(),d=read(),add(u,v,d);
    }
    dfs(1,0);
    printf("%lld\n",ans);
    return 0;
}