C++代码

//spfa
#include<iostream>
#include<algorithm>
#include<cstring>
#include<queue>

using namespace std;

const int N=1e5+10;

int n,m;
int h[N],e[N],ne[N],w[N],idx;
int dist[N];
bool vis[N];

void add(int a,int b,int c)
{
    e[idx]=b,ne[idx]=h[a],h[a]=idx,w[idx]=c;
    idx++;
}//add

int spfa()
{
    memset(dist,0x3f,sizeof dist);
    dist[1]=0;

    queue<int> q;
    q.push(1);
    vis[1]=true;

    while(q.size())
    {
        int t=q.front();
        q.pop();

        vis[t]=false;

        for(int i=h[t];i!=-1;i=ne[i])
        {
            int j=e[i];
            if(dist[j]>dist[t]+w[i])
            {
                dist[j]=dist[t]+w[i];
                if(!vis[j])
                {
                    q.push(j);
                    vis[j]=true;
                }
            }
        }
    }

    if(dist[n]==0x3f3f3f3f) return -1;
    return dist[n];
}//spfa

int main(void)
{
    scanf("%d%d",&n,&m);
    memset(h,-1,sizeof h);

    while(m--)
    {
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
    }

    int res=spfa();
    if(res==-1) puts("impossible");
    else printf("%d\n",res);

    return 0;
}