Initial commit: C++ courseware

2.Course/2.深入浅出程序设计竞赛（提高篇）/最短路/C++源码/04a_bf_negative.cpp

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+// 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';
+    }
+}