P3385 【模板】负环
1 #include <bits/stdc++.h>
2 using namespace std;
3 const int maxn = 2005, maxm = 3005;
4 const int inf = 0x3f3f3f3f;
5 struct edge {
6 int v, w;
7 };
8 vector<edge> maps[maxn];
9 int dis[maxn], n, m;
10
11 bool BellmanFord(int s) { // s为源点
12 fill(dis+1, dis+1+n, inf);
13 dis[s] = 0;
14 for (int i = 1; i <= n-1; i++) {
15 for (int u = 1; u <= n; u++) {
16 for (int j = 0; j < maps[u].size(); j++) {
17 int v = maps[u][j].v;
18 int w = maps[u][j].w;
19 if (dis[u] + w < dis[v]) {
20 dis[v] = dis[u] + w;
21 }
22 }
23 }
24 }
25
26 if (maps[s].size() == 0) return true; // 如果s点没有和其他点相连，则没有负环
27 for (int u = 1; u <= n; u++) {
28 for (int j = 0; j < maps[u].size(); j++) {
29 int v = maps[u][j].v;
30 int w = maps[u][j].w;
31 if (dis[u] + w < dis[v])
32 return false;
33 }
34 }
35 return true;
36 }
37 int main() {
38 int t; scanf("%d",&t);
39 while (t--) {
40 scanf("%d%d",&n,&m);
41 for (int i = 1; i <= n; i++) maps[i].clear();
42
43 for (int i = 1; i <= m; i++) {
44 int u, v, w; scanf("%d%d%d",&u,&v,&w);
45 if (w < 0) maps[u].push_back(edge{v,w});
46 else {
47 maps[u].push_back(edge{v,w});
48 maps[v].push_back(edge{u,w});
49 }
50 }
51 bool flag = BellmanFord(1);
52 if (flag) puts("N0");
53 else puts("YE5");
54 }
55 }
