spfa求最短路

1. dist表示每个点到源点的最短距离
2. 只有当前一个点，能更新后一个点的距离点时候，才会将该点放入堆中

#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
const int N = 100010, M = 100010, INF = 0x3f3f3f3f;
int n, m;

int h[N], e[M], v[M], ne[M], cnt;
int dist[N];
bool st[N];

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

    queue<int> q;
    q.push(1);
    st[1] = true;   // st表示是否在队列中

    while (!q.empty()) {
        auto t = q.front();
        q.pop();

        st[t] = false;
        for (int i = h[t]; ~i; i = ne[i]) {
            int j = e[i];
            if (dist[j] > dist[t] + v[i]) {
                dist[j] = dist[t] + v[i];
                if (!st[j]) {
                    q.push(j);
                    st[j] = true;
                }
            }
        }
    }
}

void add(int a, int b, int c) {
    e[cnt] = b, ne[cnt] = h[a], v[cnt] = c, h[a] = cnt++;
}

int main() {
    cin >> n >> m;
    memset(h, -1, sizeof(h));
    for (int i = 0; i < m; i++) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c);
    }

    spfa();

    if (dist[n] > INF / 2) cout << "impossible" << endl;
    else cout << dist[n] << endl;


    return 0;
}