Bellman-Ford

#include 
using namespace std;
const int N = 510, M = 1e4 + 10, INF = 0x3f3f3f3f;
int n, m, k, d[N], t[N];
struct edge{
	int u, v, w;
}e[M];
void bellman_ford(){
	memset(d, 0x3f, sizeof d);
	d[1] = 0;
	for (int i = 0; i < k; i ++ ){
		memcpy(t, d, sizeof d);
		for (int j = 0; j < m; j ++ )
			d[e[j].v] = min(d[e[j].v], t[e[j].u] + e[j].w);
	}
}
int main(){
	cin >> n >> m >> k;
	for (int i = 0; i < m; i ++ ){
		int u, v, w;
		cin >> u >> v >> w;
		e[i] = {u, v, w};
	}
	bellman_ford();
	if (d[n] > INF / 2) cout << "impossible\n";
	else cout << d[n] << "\n";
	return 0;
}

Acwing模板：https://www.acwing.com/problem/content/855/