bzoj1486

二分答案 若存在负圈则说明答案可以更优 dfs版spfa判负圈

 1 #include<bits/stdc++.h>
 2 #define clr(a,x) memset(a,x,sizeof(a))
 3 #define rep(i,l,r) for(int i=l;i<r;i++)
 4 typedef long long ll;
 5 using namespace std;
 6 int read()
 7 {
 8     char c=getchar();
 9     int ans1=0,f=1;
10     while(!isdigit(c)){
11         if(c==‘-‘) f=-1;
12         c=getchar();
13     }
14     while(isdigit(c)){
15         ans1=ans1*10+c-‘0‘;
16         c=getchar();
17     }
18     return ans1*f;
19 }
20 struct edge{
21     int to;double d,v;
22 };
23 const double esp=1e-9,inf=1e7;
24 const int maxn=3005;
25 int n,m;
26 double d[maxn];
27 bool p[maxn],flag;
28 vector<edge>e[maxn];
29 void dfs(int a)
30 {
31     p[a]=1;
32     rep(i,0,e[a].size()){
33         if(d[a]+e[a][i].v<d[e[a][i].to])
34         {
35             if(!p[e[a][i].to]){
36                 d[e[a][i].to]=d[a]+e[a][i].v;
37                 dfs(e[a][i].to);
38             }else flag=1;
39             if(flag){
40                 p[a]=0;
41                 return;
42             }
43         }
44     }
45     p[a]=0;
46 }
47 bool ok(double a)
48 {
49     clr(p,0),clr(d,0),flag=0;
50     rep(i,1,n+1){
51         rep(j,0,e[i].size()){
52             e[i][j].v=e[i][j].d-a;
53         }
54     }
55     rep(i,1,n+1){
56         dfs(i);
57         if(flag) return 1;
58     }
59     return 0;
60 }
61 double find(double l,double r)
62 {
63     if(r-l<=esp) return l;
64     double mid=(l+r)/2;
65     return ok(mid)?find(l,mid):find(mid,r);
66 }
67 int main()
68 {
69     n=read(),m=read();
70     rep(i,0,m){
71         int from=read();
72         edge ed;
73         ed.to=read();
74         scanf("%lf",&ed.d);
75         e[from].push_back(ed);
76     }
77     printf("%.8lf\n",find(-inf,inf));
78     return 0;
79 }

1486: [HNOI2009]最小圈

Time Limit: 10 Sec  Memory Limit: 64 MB
Submit: 1448  Solved: 680
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