蓝桥杯 大臣的旅费（c语言）

直接两次迪杰斯特拉（使用了堆优化，用spfa应该也能过）搜最长路径，

以样例为例，第一次以1作为起点，得到1距离4最远

第二次以4作为起点，得到4距离5最远

#include 
using namespace std;
 
typedef long long ll;
ll INF = 1e12;
const int N = 100005;
 
int n, val, max_dis;
struct node{
    int v;
    long long dis;
    node(int _v,long long _dis){
        v = _v;
        dis = _dis;
    }
    bool operator < (const node& a) const{
        return dis > a.dis;
    }
    
};
int vis[N];
long long dist[N];
vector mp[N];
 
int Dijkstra(int start)
{
    priority_queue q;
 
    for(int i = 1; i <= n; i++)
    {
        dist[i] = INF;
        vis[i] = 0;
    }
 
    dist[start]= 0;
    q.push(node(start,0));
     
    while(!q.empty())
    {
        node N = q.top();
        q.pop();
        if(vis[N.v]) continue;
        vis[N.v] = 1;
         
        for(int i = 0; i < mp[N.v].size(); i++)
        {
            node nP = mp[N.v][i];
            int to = nP.v;
            int _dis = nP.dis;
             
            if(_dis + dist[N.v] < dist[to])
            {
                dist[to] = _dis + dist[N.v];
                q.push(node(to, dist[to]));              
            }
        }  
    }
     
    int v = 0;
    for(int i = 1; i <= n; i++)
    {
        if(dist[i] > v)
            v = dist[i];
    }
    return v;
}
 
int main()
{
    int x, y, st,d;
    val = max_dis = -1;
    long long sum;
    
    scanf("%d", &n);
    
    for(int i = 1; i <= n - 1; i++)
    {
        scanf("%d %d %d", &x, &y, &d);
        mp[x].push_back(node(y, d));
        mp[y].push_back(node(x, d));
    }
    Dijkstra(1);
    for(int i = 1; i <= n; i++)
    {
        if(dist[i]>val)
        {
            val = dist[i];
            st = i;
        }
    }
    val=0;

    Dijkstra(st);
     for(int i = 1; i <= n; i++)
    {
        if(dist[i]>val)
        {
            val = dist[i];
            st = i;
        }
    }
    sum=val;
    
    cout <