bellman_ford算法（边数限制的最短路，边权可能为负）

#include <bits/stdc++.h>
using namespace std;
const int N = 550, M = 10010;
struct Edges{int a, b, w;}edges[M];
int dist[N], pre[N];
int m, n, k;

int bellman_ford() {
    memset(dist,0x3f,sizeof dist);
    dist[1] = 0;
    for(int i = 0; i < k; i++) { // 循环k次
        memcpy(pre,dist,sizeof dist);  // 备份数组
        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],pre[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 , c;
        scanf("%d%d%d",&a,&b,&c);
        edges[i] = {a,b,c};
    }
    int t = bellman_ford();
    if(t == -1) puts("impossible");
    else printf("%d\n",t);
    return 0;
}