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/04_negative_cycle.cpp

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+// P3385：检测从 1 可达的负环。C++11。
+#include <iostream>
+#include <vector>
+#include <queue>
+using namespace std;
+using ll = long long;
+struct Edge { int v, w; };
+
+bool hasNegativeCycle(const vector<vector<Edge>>& g) {
+    int n = static_cast<int>(g.size()) - 1;
+    const ll INF = (1LL << 60);
+    vector<ll> dis(n + 1, INF);
+    vector<int> len(n + 1, 0);
+    vector<bool> inq(n + 1, false);
+    queue<int> q;
+    dis[1] = 0;
+    q.push(1); inq[1] = true;
+    while (!q.empty()) {
+        int u = q.front(); q.pop();
+        inq[u] = false;
+        for (auto e : g[u]) {
+            int v = e.v;
+            ll nd = dis[u] + e.w;
+            if (nd >= dis[v]) continue;
+            dis[v] = nd;
+            len[v] = len[u] + 1;
+            if (len[v] >= n) return true;
+            if (!inq[v]) {
+                q.push(v);
+                inq[v] = 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<vector<Edge>> g(n + 1);
+        for (int i = 0; i < m; ++i) {
+            int u, v, w; cin >> u >> v >> w;
+            g[u].push_back({v, w});
+            if (w >= 0) g[v].push_back({u, w});
+        }
+        bool bad = hasNegativeCycle(g);
+        cout << (bad ? "YES" : "NO") << '\n';
+    }
+}