次短路

#include <cstdio>
#include <cstring>
#include <queue>
#include <algorithm>
using namespace std;
const int R = 100000+5;
const int INF = 0x3f3f3f3f;


struct Node
{
    int v;
    int w;
    int next;
} edge[R*2];

int head[R];
int dist1[R],dist2[R];//距离数组，分别求1到所有点距离和n到所有点距离
bool vis[R];
int num;
int n,m;

void init()
{
    num = 0;
    memset(head,-1,sizeof(head));
    memset(dist1,0x3f,sizeof(dist1));
    memset(dist2,0x3f,sizeof(dist2));
}

void add_edge(int u,int v,int w)//邻接表
{
    edge[num].v = v;
    edge[num].w = w;
    edge[num].next = head[u];
    head[u] = num++;
}

void SPFA(int u,int *dist)//u是给定点 dist是距离数组
{
    int i,v,w;
    queue<int> Q;
    memset(vis,false,sizeof(vis));
    dist[u] = 0;
    vis[u] = true;
    Q.push(u);
    while(!Q.empty())
    {
        u = Q.front();
        Q.pop();
        vis[u] = false;
        for(i=head[u]; i!=-1; i=edge[i].next)
        {
            v = edge[i].v;
            w = edge[i].w;
            if(dist[v] > dist[u] + w)
            {
                dist[v] = dist[u] + w;
                if(!vis[v])
                {
                    vis[v] = true;
                    Q.push(v);
                }
            }
        }
    }
}

int main()
{
    while(scanf("%d%d",&n,&m)!=EOF)//n个点，m条边
    {
        int i,j,u,v,w;;
        init();
        for(i=1; i<=m; i++) //无向图，双向间图
        {
            scanf("%d%d%d",&u,&v,&w);
            add_edge(u,v,w);
            add_edge(v,u,w);
        }
        SPFA(1,dist1);//求 1 到所有点的最短路
        SPFA(n,dist2);//求 n 到所有点的最短路
        int ans = INF;
        for(i=1; i<=n; i++)
        {
            for(j=head[i]; j!=-1; j=edge[j].next)
            {
                v = edge[j].v;
                w = edge[j].w;
                // 1 到 i这一点的最短路， n 到 j这一点的最短路 + edge[i][j]
                int tem = dist1[i] + dist2[v] + w;
                if(tem > dist1[n] && tem < ans)
                {
                    ans = tem;
                }
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}