观光奶牛

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<queue>
 4 using namespace std;
 5 typedef double dd;
 6 const int maxn=1007;
 7 const int maxm=5007;
 8 int n,m,num;
 9 dd l,r,mid;
10 dd d[maxn];
11 queue<int>q;
12 dd val[maxn];
13 int cnt[maxn],head[maxn];
14 bool vis[maxn];
15 struct Edge{
16     int next;int to;dd dis;
17 }edge[maxm];
18 void add(int from,int to,dd dis){
19     edge[++num].next=head[from];
20     edge[num].to=to;
21     edge[num].dis=dis;
22     head[from]=num;
23 }
24 bool check(dd mid){
25     for(int i=1;i<=n;i++){
26         q.push(i);d[i]=0;cnt[i]=1;vis[i]=true;
27     }
28     while(!q.empty()){
29         int u=q.front();q.pop();vis[u]=false;
30         for(int i=head[u];i;i=edge[i].next){
31             int v=edge[i].to;
32             if(d[v]>d[u]+(dd)mid*(dd)edge[i].dis-(dd)val[u]){
33                 d[v]=d[u]+(dd)mid*(dd)edge[i].dis-(dd)val[u];
34                 if(!vis[v]){
35                     q.push(v);vis[v]=true;
36                     if(++cnt[v]>=n) return 1;
37                 }
38             }
39         }
40     }
41     return false;
42 }
43 int main(){
44     cin>>n>>m;
45     for(int i=1;i<=n;i++) cin>>val[i];
46     for(int i=1;i<=m;i++){
47         int u,v,w;cin>>u>>v>>w;
48         add(u,v,w);
49     }
50     r=(dd)maxn*maxm;
51     while(r-l>=1e-4){
52         mid=(double)(l+r)/2;
53         if(check(mid)) l=mid;
54         else r=mid;
55     } 
56     printf("%.2lf",l);
57     return 0;
58 }