[HNOI2009]最小圈

#include<iostream>
#include<cstdio>
#include<cstring>
#define maxn 3005
#define maxe 10005
using namespace std;
struct Edge
{
	int v;
	double w;
	int next;
}e[maxe];
int head[maxn],cnts=0,n,m;
double eps=1e-9,dis[maxn];
bool vis[maxn],flag;
inline void addedge(int u,int v,double w)
{
	cnts++;
	e[cnts].v=v;
	e[cnts].w=w;
	e[cnts].next=head[u];
	head[u]=cnts;
}
void spfa(int x,double mid)
{
	vis[x]=1;
	for(int i=head[x];i!=0;i=e[i].next)
	{
		int v=e[i].v;
		if(dis[v]>dis[x]+e[i].w-mid)
		{
			if(vis[v])
			{
				flag=1;
				return;
			}
			dis[v]=dis[x]+e[i].w-mid;
			spfa(v,mid);
		}
	}
	vis[x]=0;
}
inline bool check(double mid)
{
	memset(vis,0,sizeof(vis));
	memset(dis,0,sizeof(dis));
	for(int i=1;i<=n;i++)
	{
		flag=0;
		spfa(i,mid);
		if(flag) return 1;
	}
	return 0;
}
int main()
{
	//freopen("circle.in","r",stdin);
	//freopen("circle.out","w",stdout);
	scanf("%d%d",&n,&m);
	for(int i=1,u,v;i<=m;i++)
	{
		double w;
		scanf("%d%d%lf",&u,&v,&w);
		addedge(u,v,w);
	}
	double l=-1e5,r=1e5,mid;
	while(r-l>eps)
	{
		mid=(l+r)/2;
		if(check(mid)) r=mid;
		else l=mid;
	}
	printf("%.8lf",mid);
}