最长链，两遍heap+dijiska

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
using namespace std;
const int N=40005;
int head[N],nc;
struct edge
{
    int to,cost,next;
}edge[N*3];
void add(int a,int b,int c)
{
    edge[nc].to=b;edge[nc].next=head[a];edge[nc].cost=c;head[a]=nc++;
    edge[nc].to=a;edge[nc].next=head[b];edge[nc].cost=c;head[b]=nc++;
}
struct data
{
    int id;
    int val;
    bool operator<(const data &next)const
    {
        return val<next.val;
    }
    data(int _id,int _val){id=_id;val=_val;}
    data(){}
};
int dist[N];
bool vis[N],mark[N];
priority_queue<data> Q;
int diji(int s)
{
    memset(dist,-1,sizeof(dist));
    memset(vis,false,sizeof(vis));
    dist[s]=0;
    while(!Q.empty())
        Q.pop();
    Q.push(data(s,0));
    int id,mm=-1,last=s;
    data a;
    while(!Q.empty())
    {
        a=Q.top();
        Q.pop();
        if(vis[a.id]||dist[a.id]!=a.val)
            continue;
        if(mm<a.val)
            mm=a.val,id=a.id;
        mark[a.id]=vis[a.id]=true;
        for(int i=head[a.id];i!=-1;i=edge[i].next)
        {
            int t=edge[i].to;
            if(!vis[t]&&dist[t]<dist[a.id]+edge[i].cost)
            {
                dist[t]=dist[a.id]+edge[i].cost;
                Q.push(data(t,dist[t]));
            }
        }
    }
    return id;
}
int main()
{
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        memset(head,-1,sizeof(head));
        nc=0;
        for(int i=0;i<m;i++)
        {
            int a,b,c;
            char op[3];
            scanf("%d%d%d%s",&a,&b,&c,op);
            add(a,b,c);
        }
        memset(mark,false,sizeof(mark));
        int ans=0;
        for(int i=1;i<=n;i++)
            if(!mark[i])
                ans=max(ans,dist[diji(diji(i))]);
        printf("%d\n",ans);
    }
    return 0;
}