原题地址

NOI水题，如今不用写人工栈更加水了…

AC code:

#include <cstdio>
#include <vector>
using namespace std;
const int N=1000010;
typedef long long ll;
int n;
int siz[N];
ll  ans;

struct Data{
    int v,w;

    Data() {}

    Data(int v,int w):v(v),w(w) {}
};
vector<Data> G[N];

ll Abs(int x){
    return x>0?x:-x;
}

void DFS(int x,int pre,int w){
    siz[x]=1;
    for(int i=0;i<(int)G[x].size();i++){
        int y=G[x][i].v;
        if(y==pre) continue;
        DFS(y,x,G[x][i].w);
        siz[x]+=siz[y];
    }
    ans+=w*Abs(n-(siz[x]<<1));
}

int main(){
    scanf("%d",&n);
    for(int i=1;i<n;i++){
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        G[a].push_back(Data(b,c));
        G[b].push_back(Data(a,c));
    }
    DFS(1,0,0);
    printf("%lld\n",ans);

    return 0;
}