ac 853有边数限制的最短路

#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++) {  //循环k次 是因为有backup的存在 每次从起点开始都只会扩展一个距离的边
        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);  
          // 这里用 backup[a] 的原因就是为了防止超过限制边数   如果用dist[a] 在两条边比一条边短的时候就会用两条边的距离  但是限制了边数所以不能这样
        }
    }
    if (dist[n] > 0x3f3f3f3f / 2) return 0x3f3f3f3f;
    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 == 0x3f3f3f3f) cout << "impossible";
    else cout << t;
}