[training Day12] tree! Tree! Tree! [greed] [minimum spanning tree]

Ideas ：

We enumerate the average directly , Then use the standard deviation to sort , Do a minimum spanning tree .
But the final average should be calculated by yourself

$code$

#include<algorithm>
#include<iostream>
#include<cstdio>
#include<queue>
#include<cmath>
using namespace std;
long long bj[100010],fa[100010];
double maxn,ans=1e18;
long long n,m;
struct node
{
    
	double w,b;
	int x,y;
}a[200010];
bool cmp(node&x,node&y)
{
    
	return x.b<y.b;
}
int find(int faa)
{
    
	if(fa[faa]==faa)
	  return faa;
	return fa[faa]=find(fa[faa]);
}
double kruskal()
{
    
	double js=0;
	for(int i=1; i<=n; i++)
	   fa[i]=i;
	for(int i=1; i<=m; i++)
	 {
    
	 	bj[i]=0;
	 	int fx=find(a[i].x),fy=find(a[i].y);
	 	if(fx!=fy)
	 	 {
    
	 	 	bj[i]=1,fa[fx]=fy;
	 	 	js+=a[i].w;
		 }
	 }
	double p=js/(n*1.0-1.0),anss=0;
	for(int i=1; i<=m; i++)
	 if(bj[i])
	   anss+=(a[i].w-p)*(a[i].w-p);
	return sqrt(anss/(n*1.0-1.0));
}
int main()
{
    
	scanf("%lld%lld",&n,&m);
	for(int i=1; i<=m; i++)
	 {
    
	 	scanf("%lld%lld%lf",&a[i].x,&a[i].y,&a[i].w);
	 	maxn=max(a[i].w,maxn);
	 }
	for(double i=0; i<=maxn; i+=0.1)
	 {
    
	 	for(int j=1; j<=m; j++)
	 	   a[j].b=(a[j].w-i)*(a[j].w-i);
	 	sort(a+1,a+1+m,cmp);
	 	ans=min(ans,kruskal());
	 }
	printf("%.4lf",ans);
	return 0;
}