记得备份dist数组，每次遍历所有边

#include<bits/stdc++.h>
using namespace std;
const int N = 510;
const int M = 1e4+10;
int n,m,k;
//结构体存边，包括端点和长度
struct Edge
{
    int a,b,w;
}edges[M];
int dist[N],backup[N];

int bellman_ford()
{
    memset(dist,0x3f,sizeof dist);
    dist[1] = 0;

    for(int j = 0;j<k;j++)
    {
        memcpy(backup,dist,sizeof dist);
        for(int i = 0;i<m;i++)
        {
            int a = edges[i].a,b = edges[i].b,w = edges[i].w;
            dist[b] = min(dist[b],backup[a] + w);
        }
    }
    if(dist[n] > 0x3f3f3f3f/2)return 0x3f3f3f3f;
    else return dist[n];
}

int main()
{
    cin>>n>>m>>k;
    for(int i = 0;i<m;i++)
    {
        int x,y,z;
        cin>>x>>y>>z;
        edges[i] = {x,y,z};
    }
    if(bellman_ford() == 0x3f3f3f3f)puts("impossible");
    else cout<<bellman_ford()<<endl;
    return 0;
}