【codevs 1269】匈牙利遊戲

QAQ
求次短路分三種情況討論

1、最短能更新最短
2、次短能更新次短
3、最短能更新次短並且次短不能更新最短

什麼叫不能更新最短了呢
就是更新完了和最短一樣長的
恩恩
就是這樣

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>
using namespace std;
const int MAXN = 100000 + 5;
struct edge
{
    int f,t,v;
}l[MAXN << 1];
int first[MAXN],next[MAXN],tot;
void init()
{
    memset(first,0xfff,sizeof(first));
    tot = 0;
    return;
}
void build(int f,int t,int v)
{
    l[++tot] = (edge){f,t,v};
    next[tot] = first[f];
    first[f] = tot;
    return;
}
int dis[MAXN][2];
bool use[MAXN];
queue <int> q;
void spfa(int s)
{
    memset(dis,0x3f,sizeof(dis));
    memset(use,0,sizeof(use));
    dis[s][0] = 0;
    q.push(s);
    use[s] = true;
    while(!q.empty())
    {
        int u = q.front();
        q.pop();
        use[u] = false;
        for(int i = first[u];i != -1;i = next[i])
        {
            int v = l[i].t;
            int val = l[i].v;
            if(dis[u][0] + val < dis[v][0])
            {
                dis[v][1] = dis[v][0];
                dis[v][0] = dis[u][0] + val;
                if(!use[v])
                    q.push(v);
                use[v] = true;
            }
            else if(dis[u][0] + val < dis[v][1] && dis[u][0] + val > dis[v][0])
            {
                dis[v][1] = dis[u][0] + val;
                if(!use[v])
                    q.push(v);
                use[v] = true;
            }
            else if(dis[v][1] > dis[u][1] + val)
            {
                dis[v][1] = dis[u][1] + val;
                if(!use[v])
                    q.push(v);
                use[v] = true;
            }
        }
    }
    return;
}
int n,m;
int f,t,v;
int main()
{
    init();
    scanf("%d %d",&n,&m);
    for(int i = 1;i <= m;i ++)
    {
        scanf("%d %d %d",&f,&t,&v);
        build(f,t,v);
    }
    spfa(1);
    printf("%d\n",dis[n][1] > 1000000000 ? -1 : dis[n][1]);
    return 0;
}