54 lines
1.4 KiB
C++
54 lines
1.4 KiB
C++
// P3385：检测从 1 可达的负环。C++11。
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#include <iostream>
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#include <vector>
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#include <queue>
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using namespace std;
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using ll = long long;
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struct Edge { int v, w; };
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bool hasNegativeCycle(const vector<vector<Edge>>& g) {
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int n = static_cast<int>(g.size()) - 1;
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const ll INF = (1LL << 60);
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vector<ll> dis(n + 1, INF);
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vector<int> len(n + 1, 0);
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vector<bool> inq(n + 1, false);
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queue<int> q;
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dis[1] = 0;
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q.push(1); inq[1] = true;
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while (!q.empty()) {
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int u = q.front(); q.pop();
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inq[u] = false;
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for (auto e : g[u]) {
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int v = e.v;
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ll nd = dis[u] + e.w;
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if (nd >= dis[v]) continue;
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dis[v] = nd;
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len[v] = len[u] + 1;
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if (len[v] >= n) return true;
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if (!inq[v]) {
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q.push(v);
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inq[v] = true;
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}
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}
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}
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return false;
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}
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int main() {
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ios::sync_with_stdio(false);
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cin.tie(nullptr);
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int T;
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if (!(cin >> T)) return 0;
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while (T--) {
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int n, m; cin >> n >> m;
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vector<vector<Edge>> g(n + 1);
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for (int i = 0; i < m; ++i) {
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int u, v, w; cin >> u >> v >> w;
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g[u].push_back({v, w});
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if (w >= 0) g[v].push_back({u, w});
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}
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bool bad = hasNegativeCycle(g);
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cout << (bad ? "YES" : "NO") << '\n';
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}
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}
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