spfa 算法（队列优化的Bellman-Ford算法）

#include <bits/stdc++.h>
using namespace std;

const int N = 100010;
int h[N], e[N], w[N], ne[N], idx;
int dist[N];
bool st[N];
int m, 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; // 在队列中设置为true
    while(q.size()) {
        auto t = q.front();
        q.pop();
        st[t] = false; // 出队列设置为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]) { // 不在队列中则加入
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }
    if(dist[n] == 0x3f3f3f3f) return -1;
    return dist[n];
}

int main() {
    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 t = spfa();
    if(t == - 1) puts("impossible");
    else printf("%d\n",t);
    return 0;
}