点击查看代码
#include<bits/stdc++.h>
using namespace std;const int N=3010,M=10010;
int h[N],e[M],ne[M],idx;
double w[M];
int n,m;
double dist[N];
int cnt[N],st[N];void add(int u,int v,double c)
{e[idx]=v,w[idx]=c,ne[idx]=h[u],h[u]=idx++;
}bool check(double mid)
{memset(dist,0x3f,sizeof dist);queue<int> q;for(int i=1;i<=n;i++){q.push(i);cnt[i]=0;st[i]=true;dist[i]=0;}while(q.size()){int t=q.front();q.pop();st[t]=false;for(int i=h[t];~i;i=ne[i]){int j=e[i];if(dist[j]>dist[t]+w[i]-mid){dist[j]=dist[t]+w[i]-mid;cnt[j]=cnt[t]+1;if(cnt[j]>=n) return true;if(!st[j]){q.push(j);st[j]=true;}}}}return false;
}int main()
{ios::sync_with_stdio(0),cin.tie(0);cin>>n>>m;memset(h,-1,sizeof h);double min_w=2e9,max_w=-2e9;while(m--){int i,j;double c;cin>>i>>j>>c;add(i,j,c);min_w=min(min_w,c);max_w=max(max_w,c);}double l=min_w,r=max_w;while(r-l>1e-9){double mid=l+(r-l)/2;if(check(mid)){r=mid;}else{l=mid;}}cout<<fixed<<setprecision(8)<<r<<endl;return 0;
}