NOI2003 逃学的小孩

 1 const maxn=400100;
 2 type node=record
 3      w,go,next:longint;
 4      end;
 5 var  e:array[0..maxn] of node;
 6      f:array[0..maxn,1..3] of int64;
 7      first,u:array[0..maxn] of longint;
 8      i,x,y,z,n,tot,m:longint;
 9      ans:int64;
10 function max(x,y:int64):int64;
11  begin
12  if x>y then exit(x) else exit(y);
13  end;
14 function get(x,y,z:int64):int64;
15  begin
16  get:=x+2*y+z;
17  end;
18 procedure insert(x,y,z:longint);
19  begin
20  inc(tot);e[tot].go:=y;e[tot].w:=z;e[tot].next:=first[x];first[x]:=tot;
21  end;
22 procedure init;
23  begin
24  readln(n,m);
25  for i:=1 to m do
26   begin
27   readln(x,y,z);
28   insert(x,y,z);
29   insert(y,x,z);
30   end;
31  end;
32 procedure update(x,y:int64);
33  begin
34   if y>f[x,1] then
35    begin
36    f[x,3]:=f[x,2];
37    f[x,2]:=f[x,1];
38    f[x,1]:=y;
39    end
40  else
41   if y>f[x,2] then
42    begin
43    f[x,3]:=f[x,2];
44    f[x,2]:=y;
45    end
46  else
47    f[x,3]:=max(f[x,3],y);
48  end;
49 procedure dfs1(x,fa:longint);
50  var i,y:longint;
51  begin
52  i:=first[x];
53  while i<>0 do
54   begin
55   y:=e[i].go;
56   if y<>fa then
57    begin
58    dfs1(y,x);
59    u[y]:=e[i].w;
60    update(x,f[y,1]+u[y]);
61    end;
62   i:=e[i].next;
63   end;
64  end;
65 procedure dfs2(x,fa:longint);
66  var i,y:longint;
67  begin
68  if f[x,1]+u[x]=f[fa,1] then update(x,f[fa,2]+u[x])
69  else update(x,f[fa,1]+u[x]);
70  ans:=max(ans,get(f[x,1],f[x,2],f[x,3]));
71  i:=first[x];
72  while i<>0 do
73   begin
74   y:=e[i].go;
75   if y<>fa then dfs2(y,x);
76   i:=e[i].next;
77   end;
78  end;
79 procedure main;
80  begin
81  ans:=0;
82  dfs1(1,0);
83  dfs2(1,0);
84  writeln(ans);
85  end;
86 begin
87  init;
88  main;
89 end.