图论trainning-part-1 D. Going in Cycle!!

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <cstdlib>
 5 #include <vector>
 6 #include <climits>
 7 #include <algorithm>
 8 #include <cmath>
 9 #include <queue>
10 #define LL long long
11 #define INF 0x3f3f3ff
12 using namespace std;
13 const int maxn = 60;
14 const double exps = 1e-3;
15 struct arc {
16     int to;
17     double w;
18 };
19 
20 vector<arc>g[maxn];
21 int cnt[maxn],n,m;
22 double d[maxn];
23 bool vis[maxn];
24 bool spfa() {
25     int i,j,v,u;
26     queue<int>q;
27     for(i = 1; i <= n; i++) {
28         q.push(i);
29         d[i] = INF;
30         vis[i] = true;
31         cnt[i] = 1;
32     }
33     d[1] = 0;
34     while(!q.empty()) {
35         u = q.front();
36         q.pop();
37         vis[u] = false;
38         for(i = 0; i < g[u].size(); i++) {
39             v = g[u][i].to;
40             if(d[v] > d[u]+g[u][i].w) {
41                 d[v] = d[u]+g[u][i].w;
42                 if(!vis[v]) {
43                     vis[v] = true;
44                     cnt[v]++;
45                     q.push(v);
46                     if(cnt[u] > n) return true;
47 
48                 }
49             }
50         }
51     }
52     return false;
53 }
54 void modify(double val) {
55     for(int i = 1; i <= n; i++) {
56         for(int j = 0; j < g[i].size(); j++) {
57             g[i][j].w += val;
58         }
59     }
60 }
61 bool test(double val) {
62     modify(-val);
63     bool it =  spfa();
64     modify(val);
65     return it;
66 }
67 int main() {
68     int t,i,j,u,v,k = 1;
69     double lt,rt,mid,w;
70     scanf("%d",&t);
71     while(t--) {
72         scanf("%d%d",&n,&m);
73         for(i = 0; i <= n; i++)
74             g[i].clear();
75         lt = INF;
76         rt = 0;
77         for(i = 0; i < m; i++) {
78             scanf("%d%d%lf",&u,&v,&w);
79             g[u].push_back((arc) {v,w});
80             if(lt > w) lt = w;
81             if(w > rt) rt = w;
82         }
83         printf("Case #%d: ",k++);
84         if(test(rt+1.0)) {
85             while(rt-lt > exps) {
86                 mid = lt+(rt-lt)/2;
87                 if(test(mid)) rt = mid;
88                 else lt = mid;
89             }
90             printf("%.2f\n",rt);
91         } else printf("No cycle found.\n");
92     }
93     return 0;
94 }