题解报告：poj 2631 Roads in the North（最长链）

 1 #include<iostream>
 2 #include<string.h>
 3 #include<cstdio>
 4 using namespace std;
 5 const int maxn=1e4+5;
 6 struct node{int to,cap,next;}edge[maxn<<1];
 7 int x,y,w,cnt,maxdist,head[maxn];
 8 void add_edge(int u,int v,int w){
 9     edge[cnt].to=v;
10     edge[cnt].cap=w;
11     edge[cnt].next=head[u];
12     head[u]=cnt++;
13 }
14 int dfs(int u,int fa,int &maxdist){
15     int Dmax=0,Dsec=0;
16     for(int i=head[u];~i;i=edge[i].next){
17         int v=edge[i].to;
18         if(v^fa){
19             int nowd=dfs(v,u,maxdist)+edge[i].cap;
20             if(nowd>Dmax)Dsec=Dmax,Dmax=nowd;
21             else if(nowd>Dsec)Dsec=nowd;
22         }
23     }
24     maxdist=max(maxdist,Dmax+Dsec);
25     return Dmax;
26 }
27 int main(){
28     memset(head,-1,sizeof(head));maxdist=cnt=0;
29     while(~scanf("%d %d %d",&x,&y,&w)){
30         add_edge(x,y,w);
31         add_edge(y,x,w);
32     }
33     dfs(1,-1,maxdist);
34     printf("%d\n",maxdist);
35     return 0;
36 }