codevs 1700 MST和树的重心

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<queue>
#define fo(i,a,b) for(int i=a;i<=b;i++)
#define fod(i,a,b) for(int i=a;i>=b;i--)
using namespace std;

typedef long long ll;
const int MAXN=100000;
const int MAXM=200000;
const int INF=(1<<30);

ll dis[MAXN],mini=-1,re;
int n,m,len=0,last[MAXN],fa[MAXN],pre[MAXN];
int cur,pos;
bool vis[MAXN];

struct edge{
   int u,v,w;
   friend bool operator < (const edge &x,const edge &y){
       return x.w<y.w;
   }
}e[MAXM];

int Find(int x){
   return x==fa[x]?x:fa[x]=Find(fa[x]);
}

struct Edge{
  int next,to,val;
  Edge(int n=0,int v=0,int w=0):next(n),to(v),val(w){}
}E[MAXN];

void add(int from,int to,int val){
	E[++len]=Edge(last[from],to,val);last[from]=len;
	E[++len]=Edge(last[to],from,val);last[to]=len;
}

void kruskal(){
   ll ans=0; 
   fo(i,1,n) fa[i]=i;
   sort(e+1,e+1+m);
   fo(i,1,m){
     int x=Find(e[i].u),y=Find(e[i].v);
     if(x==y) continue;
     else{
     ans+=e[i].w;
     add(e[i].u,e[i].v,e[i].w);
	 fa[x]=y;
     }
   }
   printf("%lld\n",ans);
}

void dfs(int x){
    for(int i=last[x];i;i=E[i].next){
	   int id=E[i].to;
	   if(!vis[id]){
	   	  pre[id]=x;
	      dis[id]=dis[x]+E[i].val;
	      vis[id]=true;
	      dfs(id);
	      vis[id]=false;
	   } 
	}
}

void search(int x){
    if(pre[x]) search(pre[x]);
    re=min(re,max(dis[x],mini-dis[x]));
}

int main(){
   memset(vis,0,sizeof(vis));
   scanf("%d%d",&n,&m);
   fo(i,1,m) {
   scanf("%d%d%d",&e[i].u,&e[i].v,&e[i].w);
   }
   kruskal();
   
   memset(vis,0,sizeof(vis));
   memset(dis,0,sizeof(dis));
   vis[1]=true;
   dfs(1);
   fo(i,1,n){
   if(dis[i]>mini) mini=dis[cur=i];
   }
   vis[1]=false;
   memset(pre,0,sizeof(pre));
   memset(dis,0,sizeof(dis));
   vis[cur]=true;
   dfs(cur);
   
   mini=-1;
   fo(i,1,n){
   if(dis[i]>mini) mini=dis[pos=i];
   }
   re=mini;
   search(pos);
   printf("%lld",re);
   return 0;
}