CODEVS 3023 魔法禁书目录1：追捕大妈

#include<iostream>
#include<cstdio>
#include<cstring>
#define maxv 20050
#define maxe 40050
using namespace std;
struct edge
{
    long long u,v,w,nxt;
}e[maxe];
long long n,a,b,c,g[maxv],size[maxv],nume=0,fath[maxv];
void addedge(long long u,long long v,long long w)
{
    e[++nume].u=u;
    e[nume].v=v;
    e[nume].w=w;
    e[nume].nxt=g[u];
    g[u]=nume;
}
void dfs(long long x,long long father)
{
    fath[x]=father;size[x]=1;
    for (long long i=g[x];i;i=e[i].nxt)
    {
        long long v=e[i].v;
        if (v!=father)
        {
            dfs(v,x);
            size[x]+=size[v];
        }
    }
}
int main()
{
    scanf("%lld",&n);
    for (long long i=1;i<=n-1;i++)
    {
        scanf("%lld%lld%lld",&a,&b,&c);
        addedge(a,b,c);
        addedge(b,a,c);
    }
    dfs(1,0);
    long long ans=0;
    for (long long i=1;i<=nume;i+=2)
    {
        a=e[i].u,b=e[i].v,c=e[i].w;
        if (fath[a]==b) swap(a,b);
        ans+=c*size[b]*(n-size[b]);
    }
    double now=(double)ans/(n*n-n);
    printf("%.2lf\n",now*2);
    return 0;
}