POJ 1985

求一棵树内最远的两点，DFS，顺便记录以某节点为根内最远的两点的距离，返回最远点的距离。其实是DP。

 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <cctype>
 #include <algorithm>
 #define LL unsigned __int64
 using namespace std; 

 const int N= ;

 struct Edge{
     int u,v,c;
     int next;
 }edge[N];
 int head[N],n,m,tot,ans;

 void addedge(int u,int v,int c){
     edge[tot].u=u;
     edge[tot].v=v;
     edge[tot].c=c;
     edge[tot].next=head[u];
     head[u]=tot++;
 }

 int dfs(int u,int f){
     int max1=,max2=,v,tmp;
     for(int e=head[u];e!=-;e=edge[e].next){
         v=edge[e].v;
         if(v!=f){
             tmp=edge[e].c;
             tmp+=dfs(v,u);
             if(tmp>max1){
                 max2=max1;
                 max1=tmp;
             }
             else if(tmp>max2){
                 max2=tmp;
             }
         }
     }
     if(max1+max2>ans)
     ans=max1+max2;
     return max1;
 }

 int main(){
     int u,v,c; char t;
     while(scanf("%d%d",&n,&m)!=EOF){
         tot=; ans=;
         memset(head,-,sizeof(head));
         for(int i=;i<=m;i++){
             scanf("%d %d %d %c",&u,&v,&c,&t);
         //    cout<<u<<v<<c<<endl;
             addedge(u,v,c);
             addedge(v,u,c);
         }
         dfs(,-);
         printf("%d\n",ans);
     }
     return ;
 }