POJ 1985 Cow Marathon(树的直径)

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 #define lli long long int 
 6 using namespace std;
 7 const int MAXN=100001;
 8 void read(int &n)
 9 {
10     char c=‘+‘;int x=0;bool flag=0;
11     while(c<‘0‘||c>‘9‘){c=getchar();if(c==‘-‘)flag=1;}
12     while(c>=‘0‘&&c<=‘9‘)
13     x=(x<<1)+(x<<3)+c-48,c=getchar();
14     flag==1?n=-x:n=x;
15 }
16 struct node
17 {
18     int u,v,w,nxt;
19 }edge[MAXN];
20 int head[MAXN];
21 int num=1;
22 int n,m;
23 void add_edge(int x,int y,int z)
24 {
25     edge[num].u=x;
26     edge[num].v=y;
27     edge[num].w=z;
28     edge[num].nxt=head[x];
29     head[x]=num++;
30 }
31 int dp[MAXN];
32 int ans=0;
33 int ed=0;
34 void dfs(int u,int fa,int now)
35 {
36     if(now>ans)
37     {
38         ans=now;
39         ed=u;
40     }
41     for(int i=head[u];i!=-1;i=edge[i].nxt)
42     {
43         if(edge[i].v!=fa)
44             dfs(edge[i].v,u,now+edge[i].w);
45     }
46 }
47 int  main()
48 {
49      read(n);read(m);
50      memset(head,-1,sizeof(head));
51      for(int i=1;i<=m;i++)
52      {
53          int x,y,z;char c;
54          read(x);read(y);read(z);
55          cin>>c;
56          add_edge(x,y,z);
57          add_edge(y,x,z);
58      }
59      dfs(1,0,0);
60      dfs(ed,0,0);
61      printf("%d",ans);
62     return 0;
63 }