题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=6181

       题意是有两个人比赛，第一个人一定会走最短路，问第二个人最短走多远(不能与最短路路径完全相同)。

       其实就是一个次短路问题，我用的dijkstra+链式前向星...

AC代码：

#include <bits/stdc++.h>
#define maxn 500005
#define inf 1e18
#define ll long long
using namespace std;
struct Node{
    ll to,next,w;
    bool operator < (const Node &a) const{
        return a.w < w;
    }
}Edge[maxn],Now,Next;
int head[maxn],num;
ll d1[maxn],d2[maxn];
int T,n,m;

void init(){
    memset(head,-1,sizeof(head));
    for(int i=0;i<=n;i++){
        d1[i] = inf;
        d2[i] = inf;
    }
    num = 0;
}

void add(ll u,ll v,ll w){
    Edge[num].w = w;
    Edge[num].to = v;
    Edge[num].next = head[u];
    head[u] = num ++;
}

void dijkstra(int s){
    d1[s] = 0;
    Now.to = s;
    priority_queue<Node> q;
    q.push(Now);
    while(!q.empty()){
        ll v = q.top().to;
        ll xx = q.top().w;
        q.pop();
        if(d2[v] < xx) continue;
        for(int i=head[v];i!=-1;i=Edge[i].next){
            ll u = Edge[i].to;
            ll w = Edge[i].w + xx;
            if(d1[u] > w){
                swap(d1[u], w);
                Next.w = d1[u];
                Next.to = u;
                q.push(Next);
            }
            if(d2[u] > w && d1[u] < w){
                d2[u] = w;
                Next.w = d2[u];
                Next.to = u;
                q.push(Next);
            }
        }
    }
    printf("%lld\n",d2[n]);
}

int main()
{
    scanf("%d",&T);
    while(T--){
        scanf("%d%d",&n,&m);
        init();
        for(int i=0;i<m;i++){
            ll u,v,w;
            scanf("%lld%lld%lld",&u,&v,&w);
            add(u, v, w);
            add(v, u, w);
        }
        dijkstra(1);
    }
    return 0;
}