spfa求最短路
bellman_ford()算法求最短路径

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

const int N = 510,M=10010;

int n,m,k;
int dist[N],backup[N];

struct Edge
{
    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]>0x3f3f3f3f/2) return -1;
     return dist[n];
}

int main()
{
    scanf("%d%d%d",&n,&m,&k);
    for(int i=0;i<m;i++)
    {
        int a,b,w;
        scanf("%d%d%d",&a,&b,&w);
        edges[i] = {a,b,w};
    }
    int t = bellman_ford();
    if(t==-1) puts("impossible");
    else printf("%d\n",t);
    return 0;
}