看了一些大佬的解答，还是不太明白为什么要备份数组，模拟了一下好像明白了，思考了一下午，我真是太愚蠢了。

code

#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;

const int N = 510, M = 10010, INF = 0x3f3f3f3f;

int n, m, k;
int dist[N];
int backup[N];  // 备份数组，防止串联

struct Edges{
    int a, b, w;
}edges[M];

int bellman_ford()
{
    memset(dist, 0x3f, sizeof dist);

    dist[1] = 0;

    for (int i = 0; i < k; i ++ )
    {
        memcpy(backup, dist, sizeof dist);
        for (int j = 0; j < m ; j ++ )
        {
            int a = edges[j].a, b = edges[j].b, w = edges[j].w;
            dist[b] = min(dist[b], backup[a] + w);
        }

    }

    if (dist[n] > INF / 2) return INF;
    else return dist[n];
}

int main()
{
    cin >> n >> m >> k;

    for (int i = 0; i < m; i ++ )
    {
        int a, b, w;
        cin >> a >> b >> w;
        edges[i] = {a, b, w};
    }

    int t = bellman_ford();

    if (t == INF) puts("impossible");
    else cout << t << endl;

    return 0;
}