45 lines
1.2 KiB
C++
45 lines
1.2 KiB
C++
// 教学版 Bellman-Ford 判环；输入格式与 P3385 一致。C++11。
|
|
#include <iostream>
|
|
#include <vector>
|
|
using namespace std;
|
|
using ll = long long;
|
|
struct Edge { int u, v, w; };
|
|
|
|
bool hasNegativeCycle(int n, const vector<Edge>& edges, int s) {
|
|
const ll INF = (1LL << 60);
|
|
vector<ll> dis(n + 1, INF);
|
|
dis[s] = 0;
|
|
for (int k = 1; k <= n; ++k) {
|
|
auto old = dis;
|
|
bool changed = false;
|
|
for (auto e : edges) {
|
|
if (old[e.u] == INF) continue;
|
|
ll nd = old[e.u] + e.w;
|
|
if (nd < dis[e.v]) {
|
|
dis[e.v] = nd;
|
|
changed = true;
|
|
}
|
|
}
|
|
if (!changed) return false;
|
|
if (k == n) return true;
|
|
}
|
|
return false;
|
|
}
|
|
|
|
int main() {
|
|
ios::sync_with_stdio(false);
|
|
cin.tie(nullptr);
|
|
int T;
|
|
if (!(cin >> T)) return 0;
|
|
while (T--) {
|
|
int n, m; cin >> n >> m;
|
|
vector<Edge> edges;
|
|
for (int i = 0; i < m; ++i) {
|
|
int u, v, w; cin >> u >> v >> w;
|
|
edges.push_back({u, v, w});
|
|
if (w >= 0) edges.push_back({v, u, w});
|
|
}
|
|
cout << (hasNegativeCycle(n, edges, 1) ? "YES" : "NO") << '\n';
|
|
}
|
|
}
|