#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<cstdlib>
#include<ctime>
#include<vector>
#include<queue>
#include<set>
#include<map>
#include<algorithm>
#include<utility>
using namespace std;

typedef pair<int,int> P;
const int INF=201303160;
int n,r;
vector<P> g[10000];
int d1[10000],d2[10000];
priority_queue<P,vector<P>,greater<P> > q;

void cln()
{
    memset(g,0,sizeof(g));
    for(int i=1;i<=n;i++) d1[i]=d2[i]=INF;
}

void init()
{
    for(int i=1;i<=r;i++)
    {
        int s,t,v;
        scanf("%d%d%d",&s,&t,&v);
        g[s].push_back(P(v,t));
        g[t].push_back(P(v,s));
    }
}

void dijkstra()
{
     d1[1]=0;q.push(P(0,1));
     while(!q.empty())
     {
         P pnt=q.top();q.pop();
         int v=pnt.first;
         int frm=pnt.second;
         if(v>d2[frm]) continue;//*** 
         for(int i=0;i<g[frm].size();i++)
         {
             int to=g[frm][i].second;
             int vv=g[frm][i].first+v;
             if(vv<d1[to])
             {
                 swap(d1[to],vv); //***!当之前的最小值被新值顶替时，它一样也可以参加次小值的竞争！  
                 q.push(P(d1[to],to));
             }
             if(vv<d2[to] && vv>d1[to])
             {
                 d2[to]=vv;
                 q.push(P(d2[to],to));
             }
         }
     }
}

int main()
{
    while(scanf("%d%d",&n,&r)!=EOF)
    {
        cln();
        init();
        dijkstra();
        printf("%d\n",d2[n]);
    }
    return 0;
}

cc1plus: warnings being treated as errors
In function 'void dijkstra()':
Line 48: warning: comparison between signed and unsigned integer expressions