完美

#include <iostream>
#include <queue>
#include <vector>
#include <cstring>
using namespace std;

struct node{
    int u, v;
};
int n, m;
const int N = 1e5+10;
vector<node> h[N];
int dist[N];
bool st[N];

void spfa(){
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    queue<int> q;
    q.push(1);
    st[1] = 1;
    while(q.size()){
        int t = q.front();
        st[t] = 0;
        q.pop();
        for(auto [u, v]:h[t]){
            if(dist[u] > dist[t]+v){
                dist[u] = dist[t]+v;
                if(st[u]) continue;
                st[u] = 1;
                q.push(u);
            }
        }
    }
}

int main(){
    int a, b, c;
    cin >> n >> m;
    for (int i = 1; i <= m; i ++ ){
        cin >> a >> b >> c;
        h[a].push_back({b, c});
    }

    spfa();
    if(dist[n] == 0x3f3f3f3f) cout << "impossible";
    else cout << dist[n];

    return 0;
}