feat(cses); refactor structure

codeforces/835/g.cc

@@ -0,0 +1,94 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+void solve() {
+  int n, a, b;
+  cin >> n >> a >> b;
+  vector<vector<pair<int, int>>> tree(n + 1);
+  for (int i = 0; i < n - 1; ++i) {
+    int u, v, w;
+    cin >> u >> v >> w;
+    tree[u].emplace_back(v, w);
+    tree[v].emplace_back(u, w);
+  }
+
+  unordered_set<int> xor_values;
+  vector<int> xor_a(n + 1, 0);
+  vector<bool> visited(n + 1, false);
+  bool found = false;
+  int target_xor = -1;
+
+  // First DFS to compute xor_a and check if direct a->b is 0
+  function<void(int, int)> dfs1 = [&](int u, int parent) {
+    visited[u] = true;
+    for (auto &[v, w] : tree[u]) {
+      if (v != parent && !visited[v]) {
+        xor_a[v] = xor_a[u] ^ w;
+        if (v == b) {
+          target_xor = xor_a[v];
+          if (target_xor == 0) {
+            found = true;
+          }
+        }
+        dfs1(v, u);
+      }
+    }
+  };
+
+  dfs1(a, -1);
+  if (found) {
+    cout << "YES\n";
+    return;
+  }
+
+  // Second DFS to collect xor values from a, excluding b
+  visited.assign(n + 1, false);
+  function<void(int, int)> dfs2 = [&](int u, int parent) {
+    visited[u] = true;
+    if (u != b) {
+      xor_values.insert(xor_a[u]);
+    }
+    for (auto &[v, w] : tree[u]) {
+      if (v != parent && !visited[v]) {
+        dfs2(v, u);
+      }
+    }
+  };
+
+  dfs2(a, -1);
+
+  // Third DFS to check if any node in b's subtree (or elsewhere) has xor_a[u]
+  // == target_xor
+  visited.assign(n + 1, false);
+  function<bool(int, int)> dfs3 = [&](int u, int parent) {
+    visited[u] = true;
+    if (u != a && u != b && xor_values.count(xor_a[u] ^ target_xor)) {
+      return true;
+    }
+    for (auto &[v, w] : tree[u]) {
+      if (v != parent && !visited[v]) {
+        if (dfs3(v, u)) {
+          return true;
+        }
+      }
+    }
+    return false;
+  };
+
+  if (dfs3(b, -1)) {
+    cout << "YES\n";
+  } else {
+    cout << "NO\n";
+  }
+}
+
+int main() {
+  ios_base::sync_with_stdio(false);
+  cin.tie(nullptr);
+  int t;
+  cin >> t;
+  while (t--) {
+    solve();
+  }
+  return 0;
+}