BZOJ1486: [HNOI2009]最小圈

1486: [HNOI2009]最小圈

Time Limit: 10 Sec  Memory Limit: 64 MB
Submit: 979  Solved: 473
[Submit][Status]

Description

 

题解：

湖南题为什么出个这么裸的判负环，dfs的SPFA即可，精度把握好。

代码：

 1 #include<cstdio>
 2 #include<cstdlib>
 3 #include<cmath>
 4 #include<cstring>
 5 #include<algorithm>
 6 #include<iostream>
 7 #include<vector>
 8 #include<map>
 9 #include<set>
10 #include<queue>
11 #include<string>
12 #define inf 1000000000
13 #define maxn 3005
14 #define maxm 10005
15 #define eps 1e-9
16 #define ll long long
17 #define pa pair<int,int>
18 using namespace std;
19 inline int read()
20 {
21     int x=0,f=1;char ch=getchar();
22     while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
23     while(ch>='0'&&ch<='9'){x=10*x+ch-'0';ch=getchar();}
24     return x*f;
25 }
26 struct edgs{int go,next;double w,w0;}e[maxm];
27 double d[maxn];
28 int n,m,tot,v[maxn],head[maxn];
29 bool mark[maxn],flag;
30 void insert(int x,int y,double z)
31 {
32     e[++tot].go=y;e[tot].w0=z;e[tot].next=head[x];head[x]=tot;
33 }
34 void spfa(int x)
35 {
36     if(mark[x]){flag=1;return;}
37     mark[x]=1;
38     for(int i=head[x],y;i;i=e[i].next)
39      if(d[x]+e[i].w<d[y=e[i].go])
40       {
41           d[y]=d[x]+e[i].w;
42           spfa(y);
43           if(flag)return;
44       }
45     mark[x]=0;  
46 }
47 bool check()
48 {
49     for(int i=1;i<=n;i++)d[i]=mark[i]=0;
50     flag=0;
51     for(int i=1;i<=n;i++)
52      {
53          spfa(i);
54          if(flag)return 1;
55      }
56     return 0; 
57 }
58 int main()
59 {
60     freopen("input.txt","r",stdin);
61     freopen("output.txt","w",stdout);
62     n=read();m=read();
63     int x,y;double z,l=inf,r=-inf,mid;
64     while(m--)
65      {
66       x=read();y=read();scanf("%lf",&z);insert(x,y,z);
67       l=min(l,z);
68       r=max(r,z);
69      }
70     while(r-l>=eps)
71      {
72          mid=(l+r)/2;
73          for(int i=1;i<=tot;i++)e[i].w=e[i].w0-mid;
74          if(check())r=mid;else l=mid;
75      }
76     printf("%.8lf\n",l);  
77     return 0;
78 }

View Code