feat: Add TwoEdgeConnectedComponents implementation and test case

Author: weilycoder

test/two_edge_connected_components.test.cpp

@@ -0,0 +1,31 @@
+#define PROBLEM "https://judge.yosupo.jp/problem/two_edge_connected_components"
+
+#include "../weilycoder/graph/tarjan.hpp"
+#include <iostream>
+using namespace std;
+using namespace weilycoder;
+
+int main() {
+  cin.tie(nullptr)->sync_with_stdio(false);
+  cin.exceptions(cin.failbit | cin.badbit);
+  size_t n, m;
+  cin >> n >> m;
+
+  TwoEdgeConnectedComponents graph(n);
+  for (size_t i = 0; i < m; ++i) {
+    size_t u, v;
+    cin >> u >> v;
+    graph.add_edge(u, v);
+  }
+
+  graph.solve();
+
+  cout << graph.eccs.size() << '\n';
+  for (const auto &ecc : graph.eccs) {
+    cout << ecc.size();
+    for (size_t u : ecc)
+      cout << ' ' << u;
+    cout << '\n';
+  }
+  return 0;
+}

weilycoder/graph/tarjan.hpp

@@ -0,0 +1,61 @@
+#ifndef WEILYCODER_TARJAN_HPP
+#define WEILYCODER_TARJAN_HPP
+
+#include <cstddef>
+#include <stack>
+#include <utility>
+#include <vector>
+
+namespace weilycoder {
+template <typename ptr_t = size_t> struct TwoEdgeConnectedComponents {
+  ptr_t dfs_time = 0, edge_time = 1;
+
+  std::vector<bool> in_stack;
+  std::stack<ptr_t> stk;
+  std::vector<ptr_t> dfn, low;
+  std::vector<std::vector<std::pair<ptr_t, ptr_t>>> graph;
+
+  std::vector<std::vector<ptr_t>> eccs;
+
+  TwoEdgeConnectedComponents(ptr_t n)
+      : in_stack(n, false), dfn(n, 0), low(n, 0), graph(n) {}
+
+  void add_edge(ptr_t u, ptr_t v) {
+    graph[u].emplace_back(v, edge_time);
+    graph[v].emplace_back(u, edge_time);
+    edge_time++;
+  }
+
+  void tarjan(ptr_t u, ptr_t parent_edge) {
+    dfn[u] = low[u] = ++dfs_time;
+    stk.push(u);
+    in_stack[u] = true;
+
+    for (const auto &[v, edge_id] : graph[u]) {
+      if (edge_id == parent_edge)
+        continue;
+      if (!dfn[v])
+        tarjan(v, edge_id), low[u] = std::min(low[u], low[v]);
+      else if (in_stack[v])
+        low[u] = std::min(low[u], dfn[v]);
+    }
+
+    if (dfn[u] == low[u]) {
+      eccs.emplace_back();
+      eccs.back().push_back(u);
+      in_stack[u] = false;
+      while (stk.top() != u)
+        eccs.back().push_back(stk.top()), in_stack[stk.top()] = false, stk.pop();
+      stk.pop();
+    }
+  }
+
+  void solve() {
+    for (size_t i = 0; i < graph.size(); ++i)
+      if (!dfn[i])
+        tarjan(i, 0);
+  }
+};
+} // namespace weilycoder
+
+#endif