poj3255

#include<iostream>
#include<cstdio>
#include<cstring>
#include<queue>
#include<vector>
#include<map>
#include<algorithm>
using namespace std;
#define INF 0x3f3f3f3f
struct edge
{
    int to,cost;
 };
typedef pair<int,int> P;//first是最短距离，second是顶点的编号 
 int n,r;
 vector<edge>G[100005];
 int d[100005];
 int dist[100005],dist2[100005];//最短路和次短路 
 
 void dijkstra()
 {
     priority_queue<P,vector<P>,greater<P> >que;//从小到大的优先队列 
     fill(dist,dist+n,INF);
     fill(dist2,dist2+n,INF);
     dist[0]=0; 
     que.push(P(0,0));
     
     while(!que.empty())
     {
         P q=que.top();    que.pop();
         int v=q.second,d=q.first;
         if(dist2[v]<d)
             continue;
         for(int i=0;i<G[v].size();i++)
         {
             edge &e=G[v][i];
             int d2=d+e.cost;
            if(dist[e.to]>d2)
            {
                swap(dist[e.to],d2);
                que.push(P(dist[e.to],e.to));
            } 
            if(dist2[e.to]>d2&&dist[e.to]<d2)
            {
                dist2[e.to]=d2;
                que.push(P(dist2[e.to],e.to));
            }
         }
    }
    cout<<dist2[n-1]<<endl;
 }
 
 int main()
 {
     int a,b,c;
     while(cin>>n>>r)
     {
         for(int i=0;i<n;i++)
             G[i].clear(); 
         for(int i=0;i<r;i++)
         {
             scanf("%d%d%d",&a,&b,&c);
             a--;
             b--;
             G[a].push_back(edge{b,c});//存放边 //在结构体重存数用花括号
             G[b].push_back(edge{a,c});//无向边 
         }
         dijkstra();
     }
     return 0;
 }