HDU2376——Average distance（思维+树形DP）

#pragma GCC optimize(3)
#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline")
#pragma GCC optimize(2)
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll>PLL;
typedef pair<int,int>PII;
typedef pair<double,double>PDD;
#define I_int ll
inline ll read()
{
    ll x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
char F[200];
inline void out(I_int x) {
    if (x == 0) return (void) (putchar('0'));
    I_int tmp = x > 0 ? x : -x;
    if (x < 0) putchar('-');
    int cnt = 0;
    while (tmp > 0) {
        F[cnt++] = tmp % 10 + '0';
        tmp /= 10;
    }
    while (cnt > 0) putchar(F[--cnt]);
    //cout<<" ";
}
ll ksm(ll a,ll b,ll p){ll res=1;while(b){if(b&1)res=res*a%p;a=a*a%p;b>>=1;}return res;}
const int inf=0x3f3f3f3f,mod=998244353;
const ll INF = 0x3f3f3f3f3f3f3f3f;
const int maxn=1e6+100,maxm=3e5+7,N=1e6+7;
const double PI = atan(1.0)*4;
int h[maxn],idx,n;
struct node{
    int e,ne,w;
}edge[maxn*2];
void add(int u,int v,int w){
    edge[idx]={v,h[u],w};h[u]=idx++;
}
ll res=0;
int cnt[maxn];
void dfs(int u,int fa){
    cnt[u]=1;
    for(int i=h[u];~i;i=edge[i].ne){
        int j=edge[i].e,w=edge[i].w;
        if(j==fa) continue;
        dfs(j,u);
        cnt[u]+=cnt[j];
        res=res+(ll)(n-cnt[j])*cnt[j]*w;
    }
}
int main(){
    int T=read();
    while(T--){
        n=read();
        memset(h,-1,sizeof h);idx=0;res=0;
        for(int i=1;i<n;i++){
            int u=read(),v=read(),w=read();
            add(u,v,w);add(v,u,w);
        }
        dfs(1,-1);
        int tt=n*(n-1)/2;
        printf("%.6f\n",(double)res/tt);
    }
    return 0;
}