895

codeforces/898/h.cc

@@ -0,0 +1,100 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+void YES() {
+  cout << "YES\n";
+}
+void NO() {
+  cout << "NO\n";
+}
+
+void solve() {
+  int n, a, b;
+  cin >> n >> a >> b;
+  vector<vector<int>> graph(n + 1);
+  for (int i = 0; i < n; ++i) {
+    int u, v;
+    cin >> u >> v;
+    graph[u].push_back(v);
+    graph[v].push_back(u);
+  }
+
+  vector<int> parent(n + 1, -1);
+  vector<int> visited(n + 1, 0);
+  int cycle_start = -1, cycle_end = -1;
+  function<bool(int, int)> dfs_cycle = [&](int u, int par) {
+    visited[u] = 1;
+    for (int v : graph[u]) {
+      if (v == par)
+        continue;
+      if (visited[v] == 1) {
+        cycle_start = v;
+        cycle_end = u;
+        return true;
+      }
+      parent[v] = u;
+      if (dfs_cycle(v, u))
+        return true;
+    }
+    return false;
+  };
+
+  dfs_cycle(1, -1);
+
+  vector<int> cycle_nodes;
+  if (cycle_start != -1) {
+    for (int u = cycle_end; u != cycle_start; u = parent[u]) {
+      cycle_nodes.push_back(u);
+    }
+    cycle_nodes.push_back(cycle_start);
+  }
+
+  unordered_set<int> cycle(cycle_nodes.begin(), cycle_nodes.end());
+
+  auto bfs_dist = [&](int start) -> vector<int> {
+    vector<int> dist(n + 1, -1);
+    queue<int> q;
+    dist[start] = 0;
+    q.push(start);
+    while (!q.empty()) {
+      int u = q.front();
+      q.pop();
+      for (int v : graph[u]) {
+        if (dist[v] == -1) {
+          dist[v] = dist[u] + 1;
+          q.push(v);
+        }
+      }
+    }
+    return dist;
+  };
+
+  vector<int> dist_a = bfs_dist(a);
+  vector<int> dist_b = bfs_dist(b);
+
+  bool can_avoid = false;
+  for (int node : cycle_nodes) {
+    if (dist_b[node] < dist_a[node]) {
+      can_avoid = true;
+      break;
+    }
+  }
+
+  if (can_avoid) {
+    YES();
+  } else {
+    NO();
+  }
+}
+
+int main() {
+  ios_base::sync_with_stdio(false);
+  cin.tie(nullptr);
+
+  int t;
+  cin >> t;
+  while (t--) {
+    solve();
+  }
+  return 0;
+}