Pku1985 cow marathon

Find the diameter of a tree. The question does not contain the data range. In any case, it is obviously O (n.

DFS is used here. First, find the farthest point of any point, and then find the farthest point from this farthest point. The last two points are the answer.

View code

 1 program pku1985(input,output);
 2 type
 3    node    = ^link;
 4    link    = record 
 5          goal,w : longint;
 6          next   : node;
 7       end;        
 8 var
 9    l   : array[0..50000] of node;
10    d   : array[0..50000] of longint;
11    v   : array[0..50000] of boolean;
12    n,m : longint;
13 procedure add(xx,yy,ww :longint );
14 var
15    tt : node;
16 begin
17    new(tt);
18    tt^.next:=l[xx];
19    tt^.goal:=yy;
20    tt^.w:=ww;
21    l[xx]:=tt;
22 end; { add }
23 procedure init;
24 var
25    i,xxx,yyy,www : longint;
26 begin
27    readln(n,m);
28    for i:=1 to n do
29       l[i]:=nil;
30    for i:=1 to m do
31    begin
32       readln(xxx,yyy,www);
33       add(xxx,yyy,www);
34       add(yyy,xxx,www);
35    end;
36 end; { init }
37 procedure dfs(now,dist : longint );
38 var
39    t : node;
40 begin
41    d[now]:=dist;
42    t:=l[now];
43    while t<>nil do
44    begin
45       if not v[t^.goal] then
46       begin
47      v[t^.goal]:=true;
48      dfs(t^.goal,dist+t^.w);
49       end;
50       t:=t^.next;
51    end;
52 end; { dfs }
53 procedure main;
54 var
55    i,maxn,maxl : longint;
56 begin
57    fillchar(d,sizeof(d),0);
58    fillchar(v,sizeof(v),false);
59    v[1]:=true;
60    dfs(1,0);
61    maxl:=0;
62    for i:=1 to n do
63       if d[i]>maxl then
64       begin
65      maxl:=d[i];
66      maxn:=i;
67       end;
68    fillchar(d,sizeof(d),0);
69    fillchar(v,sizeof(v),false);
70    v[maxn]:=true;
71    dfs(maxn,0);
72    maxl:=0;
73    for i:=1 to n do
74       if d[i]>maxl then
75      maxl:=d[i];
76    writeln(maxl);
77 end; { main }
78 begin
79    init;
80    main;
81 end.