851. spfa求最短路

Posted 2021-05-14 幽殇默

https://www.acwing.com/problem/content/853/

#include<cstdio>
#include<iostream>
#include<cstring>
#include<queue>
using namespace std;
const int N=1e5+10;
int h[N],e[N],ne[N],w[N],idx;
int n,m;
int dist[N];
bool st[N];
void add(int a,int b,int c)
{
    e[idx]=b,w[idx]=c,ne[idx]=h[a],h[a]=idx++;
}
int spfa()
{
    memset(dist,0x3f,sizeof dist);
    dist[1]=0;
    
    queue<int> q; q.push(1);
    st[1]=true;
    
    while(q.size())
    {
        int t=q.front(); q.pop();
        st[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(!st[j]) 
                {
                    st[j]=true;
                    q.push(j);
                }
            }
        }
    }
    if(dist[n]==0x3f3f3f3f) return -1;
    else return dist[n];
}
int main(void)
{
    memset(h,-1,sizeof h);
    scanf("%d%d",&n,&m);
    while(m--)
    {
        int a,b,c; scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);
    }
    if(spfa()==-1) puts("impossible");
    else cout<<spfa()<<endl;
    return 0;
}