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| #include<algorithm>
#include<iostream>
#include<iomanip>
#include<cstring>
#include<cstdlib>
#include<climits>
#include<vector>
#include<cstdio>
#include<cmath>
#include<queue>
using namespace std;
inline const int Get_Int() {
int num=0,bj=1;
char x=getchar();
while(x<'0'||x>'9') {
if(x=='-')bj=-1;
x=getchar();
}
while(x>='0'&&x<='9') {
num=num*10+x-'0';
x=getchar();
}
return num*bj;
}
const int maxn=100005;
struct Edge {
int from,to,dist;
};
int n,m,root,Degree[maxn];
double f[maxn],d[maxn];
bool onCircle[maxn];
vector<Edge> edges[maxn];
void AddEdge(int x,int y,int v) {
edges[x].push_back((Edge) {x,y,v});
}
void Dfs1(int Now,int father) {
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==father||onCircle[Next])continue;
Degree[Now]++;
Dfs1(Next,Now);
d[Now]+=f[Next]+e.dist;
}
if(Degree[Now])f[Now]=d[Now]/Degree[Now];
if(Now!=root)Degree[Now]++;
}
void Dfs2(int Now,int father) {
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==father||onCircle[Next])continue;
d[Next]+=(d[Now]-f[Next]-e.dist)/max(1,Degree[Now]-1)+e.dist;
Dfs2(Next,Now);
}
}
namespace Tree {
void solve() {
root=1;
Dfs1(1,0);
Dfs2(1,0);
}
}
namespace CircleAndTree {
int Dfn[maxn],father[maxn],step=0;
double g[maxn],h[maxn];
bool bj=0;
void Find_Circle(int Now) {
Dfn[Now]=++step;
for(Edge& e:edges[Now]) {
int Next=e.to;
if(!Dfn[Next]) {
father[Next]=Now;
Find_Circle(Next);
} else if(father[Now]!=Next&&Dfn[Next]<Dfn[Now]) {
onCircle[Next]=1;
while(Now!=Next) {
onCircle[Now]=1;
Now=father[Now];
}
bj=1;
return;
}
if(bj)return;
}
}
void Dfs(int Now,int father) {
bool bj=0;
g[Now]=0;
for(Edge& e:edges[Now]) {
int Next=e.to;
if(Next==root||Next==father||!onCircle[Next])continue;
bj=1;
Dfs(Next,Now);
g[Now]+=g[Next]+e.dist;
}
if(Now==root)return;
if(!bj)g[Now]=d[Now]/max(Degree[Now],1);
else g[Now]=(g[Now]+d[Now])/(Degree[Now]+1);
}
void solve() {
Find_Circle(1);
for(int i=1; i<=n; i++)
if(onCircle[i]) {
root=i;
Dfs1(i,0);
}
for(int i=1; i<=n; i++)
if(onCircle[i]) {
root=i;
Dfs(i,0);
h[i]=g[i];
}
for(int i=1; i<=n; i++)
if(onCircle[i]) {
Degree[i]+=2;
d[i]+=h[i];
}
for(int i=1; i<=n; i++)
if(onCircle[i])Dfs2(i,0);
}
}
int main() {
n=Get_Int();
m=Get_Int();
for(int i=1; i<=m; i++) {
int x=Get_Int(),y=Get_Int(),v=Get_Int();
AddEdge(x,y,v);
AddEdge(y,x,v);
}
if(m==n-1)Tree::solve();
else CircleAndTree::solve();
double ans=0;
for(int i=1; i<=n; i++)ans+=d[i]/Degree[i];
printf("%0.5lf\n",ans/n);
return 0;
}
|
