1486: [HNOI2009]最小圈 - BZOJ

 1 const
 2     maxn=3010;
 3     maxm=10010;
 4     inf=99999;
 5 var
 6     first:array[0..maxn]of longint;
 7     next,last:array[0..maxm]of longint;
 8     w:array[0..maxm]of double;
 9     n,m:longint;
10 
11 procedure init;
12 var
13     i,x:longint;
14 begin
15     read(n,m);
16     for i:=1 to m do
17       begin
18         read(x);
19         read(last[i],w[i]);
20         next[i]:=first[x];
21         first[x]:=i;
22       end;
23 end;
24 
25 var
26     flag,vis:array[0..maxn]of longint;
27     dis:array[0..maxn]of double;
28     time,time2:longint;
29     ans:double;
30 
31 function dfs(x:longint;f:double):boolean;
32 var
33     i:longint;
34 begin
35     flag[x]:=time;
36     vis[x]:=time2;
37     dis[x]:=f;
38     i:=first[x];
39     while i<>0 do
40       begin
41         if f+w[i]-ans<=0 then
42         begin
43           if vis[last[i]]=time2 then
44             if dis[last[i]]>=f+w[i]-ans then exit(true);
45           if vis[last[i]]<>time2 then
46             if dfs(last[i],f+w[i]-ans) then exit(true);
47         end;
48         i:=next[i];
49       end;
50     vis[x]:=time2-1;
51     exit(false);
52 end;
53 
54 procedure work;
55 var
56     i:longint;
57     l,r:double;
58     f:boolean;
59 begin
60     l:=-10000001;
61     r:=-l;
62     while r-l>1e-9 do
63       begin
64         ans:=(l+r)/2;
65         inc(time);
66         f:=false;
67         for i:=1 to n do
68           if flag[i]<>time then
69           begin
70             inc(time2);
71             if dfs(i,0) then
72             begin
73               f:=true;
74               break;
75             end;
76           end;
77         if f then r:=ans
78         else l:=ans;
79       end;
80     writeln(ans:0:8);
81 end;
82 
83 begin
84     init;
85     work;
86 end.