Highway 湘潭邀请赛（江苏省赛）----树的直径

#include <cstdio>
#include <iostream>
#include <cstring>
#include <queue>
#define inf 0x3f3f3f3f
#define LL long long
using namespace std;
const int maxn=100100;
struct node{
int u,v,w;
int pre;
}edge[maxn<<1];
int nEdge,p[maxn];
LL dis[maxn];
LL DD[maxn];
bool vis[maxn];
void Init(){
memset(dis,0,sizeof(dis));
memset(p,-1,sizeof(p));
memset(vis,false,sizeof(vis));
nEdge=0;
}
void connect(int u,int v,int w){
edge[nEdge].u=u;
edge[nEdge].w=w;
edge[nEdge].v=v;
edge[nEdge].pre=p[u];
p[u]=nEdge++;

edge[nEdge].u=v;
edge[nEdge].w=w;
edge[nEdge].v=u;
edge[nEdge].pre=p[v];
p[v]=nEdge++;
}
int bfs(int s){
queue<int >q;
while(!q.empty())
q.pop();
vis[s]=true;
q.push(s);
LL maxx=0;
int key=-1;
while(!q.empty()){
int t=q.front();
//cout<<t<<endl;
q.pop();
for(int i=p[t];~i;i=edge[i].pre){
int v=edge[i].v;
if(!vis[v]&&dis[v]<dis[t]+edge[i].w){
vis[v]=true;
dis[v]=dis[t]+edge[i].w;
q.push(v);
if(dis[v]>maxx){
maxx=dis[v];
key=v;
}
}
}
}
return key;
}
void bfs1(int s){
queue<int >q;
while(!q.empty())
q.pop();
vis[s]=true;
q.push(s);
while(!q.empty()){
int t=q.front();
q.pop();
for(int i=p[t];~i;i=edge[i].pre){
int v=edge[i].v;
if(!vis[v]&&dis[v]<dis[t]+edge[i].w){
vis[v]=true;
dis[v]=dis[t]+edge[i].w;
DD[v]=max(DD[v],dis[v]);
q.push(v);
}
}
}
}
int main(){
int n;
while(~scanf("%d",&n)){
Init();
memset(DD,0,sizeof(DD));
for(int i=0;i<n-1;i++){
int u,v,w;
scanf("%d%d%d",&u,&v,&w);
connect(u,v,w);
}
//for(int i=1;i<=n;i++){
//    printf("%d ",DD[i]);
//}
//cout<<endl;
int a=bfs(1);
memset(vis,false,sizeof(vis));
memset(dis,0,sizeof(dis));
int b=bfs(a);
LL sum=dis[b];
//cout<<a<<" "<<b<<endl;
memset(vis,false,sizeof(vis));
memset(dis,0,sizeof(dis));
bfs1(a);
//cout<<"--------"<<endl;
memset(vis,false,sizeof(vis));
memset(dis,0,sizeof(dis));
bfs1(b);
//for(int i=1;i<=n;i++){
//    printf("%d ",DD[i]);
//}
//cout<<endl;
for(int i=1;i<=n;i++){
if(i==a||i==b)
continue;
sum+=DD[i];
}
cout<<sum<<endl;
}
return 0;
}

/*
5
1 2 2
1 3 1
2 4 2
3 5 1
*/