【BZOJ1509】逃学的小孩

/*To The End Of The Galaxy*/
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<queue>
#include<iomanip>
#include<stack>
#include<map>
#include<time.h>
#include<set>
#include<cmath>
#define debug(x) cerr<<#x<<"="<<x<<endl
#define INF 0x7f7f7f7f
#define llINF 0x7fffffffffffll
#define P(x,y) (((x-1)*m)+y)
using namespace std;
typedef pair<int,int> pii;
typedef long long ll;
inline int init()
{
    int now=0,ju=1;char c;bool flag=false;
    while(1)
    {
        c=getchar();
        if(c==‘-‘)ju=-1;
        else if(c>=‘0‘&&c<=‘9‘)
        {
            now=now*10+c-‘0‘;
            flag=true;
        }
        else if(flag)return now*ju;
    }
}
inline long long llinit()
{
    long long now=0,ju=1;char c;bool flag=false;
    while(1)
    {
        c=getchar();
        if(c==‘-‘)ju=-1;
        else if(c>=‘0‘&&c<=‘9‘)
        {
            now=now*10+c-‘0‘;
            flag=true;
        }
        else if(flag)return now*ju;
    }
}
struct edge
{
    int from,to,val,pre;
}Edge[1000005];
int head[200005];
ll dis[200005];
bool vis[200005];
ll d[200005];
ll dist=0;
int cnt=0,A,B;
inline void addedge(int from,int to,int val)
{
    ++cnt;
    Edge[cnt]=((edge){from,to,val,head[from]});
    head[from]=cnt;
}
queue<int> q;
int n,m;
void bfs()
{
    int now;
    q.push(1);dis[1]=0;
    while(!q.empty())
    {
        now=q.front();q.pop();
        if(vis[now])continue;
        vis[now]=1;
        for(int j=head[now];j;j=Edge[j].pre)
        {
            if(!vis[Edge[j].to])
            {
                dis[Edge[j].to]=dis[now]+Edge[j].val;
                q.push(Edge[j].to);
            }
        }
    }
    ll nowmax=0;
    for(int i=1;i<=n;i++)
    {
        if(dis[i]>nowmax)
        {
            now=i;nowmax=dis[i];
        }
    }
    A=now;
    memset(vis,0,sizeof(vis));
    q.push(now);dis[now]=0;
    while(!q.empty())
    {
        now=q.front();q.pop();
        if(vis[now])continue;
        vis[now]=1;
        for(int j=head[now];j;j=Edge[j].pre)
        {
            if(!vis[Edge[j].to])
            {
                dis[Edge[j].to]=dis[now]+Edge[j].val;
                q.push(Edge[j].to);
            }
        }
    }    
    for(int i=1;i<=n;i++)
    {
        if(dis[i]>dist)
        {
            B=i;
            dist=dis[i];
        }
    }
    now=B;
    memset(vis,0,sizeof(vis));
    q.push(now);d[now]=0;
    while(!q.empty())
    {
        now=q.front();q.pop();
        if(vis[now])continue;
        vis[now]=1;
        for(int j=head[now];j;j=Edge[j].pre)
        {
            if(!vis[Edge[j].to])
            {
                d[Edge[j].to]=d[now]+Edge[j].val;
                q.push(Edge[j].to);
            }
        }
    }    
}
#ifdef unix
    #define LLD "%lld"
#else
    #define LLD "%I64d"
#endif
int main()
{
    ll ans=0;
    int a,b,c;
    n=init();m=init();
    for(int i=1;i<=m;i++)
    {
        a=init();b=init();c=init();
        addedge(a,b,c);
        addedge(b,a,c);
    }
    bfs();
    for(int i=1;i<=n;i++)
    {
        if(i==A||i==B)continue;
        else
        {
            ans=max(ans,min(dis[i],d[i])+dist);
        }
    }
    printf(LLD"\n",ans);
    return 0;
}