graph/find_even_cycle.hpp

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• Last update: 2024-12-25 20:50:37+09:00
• Include: #include "graph/find_even_cycle.hpp"
• Link: https://chaoxu.prof/posts/2014-03-08-even-cycle-in-a-simple-graph.html
• Link: https://scoreboard.icpc.global/46/finals46.pdf

Depends on

• ds/hashmap.hpp
• graph/base.hpp

Verified with

• test/1_mytest/find_even_cycle.test.cpp

Code

#include "graph/base.hpp"

// (vs, es), size=(n+1,n)
// [R] https://scoreboard.icpc.global/46/finals46.pdf
template <typename GT>
pair<vc<int>, vc<int>> find_even_cycle(GT &G) {
  /*
  参考：https://chaoxu.prof/posts/2014-03-08-even-cycle-in-a-simple-graph.html
  DFS木を作る, 後退辺ごとにサイクルを作る
  偶サイクルができれば OK
  共通辺を持つ奇サイクルがある → それらの XOR が満たす
  どちらもない → 奇サイクルからなるカクタス → むり
  */
  static_assert(!GT::is_directed);
  const int N = G.N, M = G.M;
  if (N == 1) return {{}, {}};
  bool FOUND = 0;
  vvc<int> cyc; // edge index
  vc<int> in_cycle(M, -1);
  vc<int> path_v;
  vc<int> path_e;
  vc<bool> used(M);
  vc<bool> vis(N);
  pair<int, int> cid = {-1, -1};
  auto dfs = [&](auto &dfs, int v) -> void {
    for (auto &e: G[v]) {
      if (FOUND) return;
      if (used[e.id]) continue;
      used[e.id] = 1;

      if (!vis[e.to]) {
        vis[e.to] = 1, used[e.id] = 1;
        path_v.eb(e.to), path_e.eb(e.id);
        dfs(dfs, e.to);
        if (FOUND) return;
        POP(path_v), POP(path_e);
        continue;
      }
      // cycle 発見
      vc<int> EID = {e.id};
      int p = len(path_v) - 1;
      while (path_v[p] != e.to) { EID.eb(path_e[--p]); }
      int k = len(cyc);
      cyc.eb(EID);
      if (len(EID) % 2 == 0) {
        cid = {k, k}, FOUND = 1;
        return;
      }
      for (auto &i: EID) {
        if (in_cycle[i] != -1) {
          cid = {k, in_cycle[i]}, FOUND = 1;
          return;
        }
        in_cycle[i] = k;
      }
    }
  };
  FOR(v, N) {
    if (vis[v]) continue;
    path_v = {int(v)};
    vis[v] = 1;
    dfs(dfs, v);
  }
  if (!FOUND) return {{}, {}};
  vc<int> es = [&]() {
    if (cid.fi == cid.se) { return cyc[cid.fi]; }
    auto [a, b] = cid;
    vc<int> use(M);
    for (auto &e: cyc[a]) use[e] ^= 1;
    for (auto &e: cyc[b]) use[e] ^= 1;
    vc<int> es;
    FOR(i, M) if (use[i]) es.eb(i);
    return es;
  }();

  vc<pair<int, int>> v_to_e(N, {-1, -1});
  auto add = [&](int v, int e) -> void {
    if (v_to_e[v].fi == -1) {
      v_to_e[v].fi = e;
    } else {
      v_to_e[v].se = e;
    }
  };
  for (auto &e: es) { add(G.edges[e].frm, e), add(G.edges[e].to, e); }

  int v0 = G.edges[es[0]].frm;
  vc<int> vs = {v0};
  es.clear(), es.shrink_to_fit();
  es = {v_to_e[v0].fi};
  while (1) {
    int e = es.back();
    int a = G.edges[e].frm, b = G.edges[e].to;
    assert(vs.back() == a || vs.back() == b);
    if (vs.back() != a) swap(a, b);
    vs.eb(b);
    if (b == v0) break;
    es.eb(v_to_e[b].fi + v_to_e[b].se - e);
  }
  return {vs, es};
}

#line 2 "ds/hashmap.hpp"

// u64 -> Val

template <typename Val>
struct HashMap {
  // n は入れたいものの個数で ok

  HashMap(u32 n = 0) { build(n); }
  void build(u32 n) {
    u32 k = 8;
    while (k < n * 2) k *= 2;
    cap = k / 2, mask = k - 1;
    key.resize(k), val.resize(k), used.assign(k, 0);
  }

  // size を保ったまま. size=0 にするときは build すること.

  void clear() {
    used.assign(len(used), 0);
    cap = (mask + 1) / 2;
  }
  int size() { return len(used) / 2 - cap; }

  int index(const u64& k) {
    int i = 0;
    for (i = hash(k); used[i] && key[i] != k; i = (i + 1) & mask) {}
    return i;
  }

  Val& operator[](const u64& k) {
    if (cap == 0) extend();
    int i = index(k);
    if (!used[i]) { used[i] = 1, key[i] = k, val[i] = Val{}, --cap; }
    return val[i];
  }

  Val get(const u64& k, Val default_value) {
    int i = index(k);
    return (used[i] ? val[i] : default_value);
  }

  bool count(const u64& k) {
    int i = index(k);
    return used[i] && key[i] == k;
  }

  // f(key, val)

  template <typename F>
  void enumerate_all(F f) {
    FOR(i, len(used)) if (used[i]) f(key[i], val[i]);
  }

private:
  u32 cap, mask;
  vc<u64> key;
  vc<Val> val;
  vc<bool> used;

  u64 hash(u64 x) {
    static const u64 FIXED_RANDOM = std::chrono::steady_clock::now().time_since_epoch().count();
    x += FIXED_RANDOM;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return (x ^ (x >> 31)) & mask;
  }

  void extend() {
    vc<pair<u64, Val>> dat;
    dat.reserve(len(used) / 2 - cap);
    FOR(i, len(used)) {
      if (used[i]) dat.eb(key[i], val[i]);
    }
    build(2 * len(dat));
    for (auto& [a, b]: dat) (*this)[a] = b;
  }
};
#line 3 "graph/base.hpp"

template <typename T>
struct Edge {
  int frm, to;
  T cost;
  int id;
};

template <typename T = int, bool directed = false>
struct Graph {
  static constexpr bool is_directed = directed;
  int N, M;
  using cost_type = T;
  using edge_type = Edge<T>;
  vector<edge_type> edges;
  vector<int> indptr;
  vector<edge_type> csr_edges;
  vc<int> vc_deg, vc_indeg, vc_outdeg;
  bool prepared;

  class OutgoingEdges {
  public:
    OutgoingEdges(const Graph* G, int l, int r) : G(G), l(l), r(r) {}

    const edge_type* begin() const {
      if (l == r) { return 0; }
      return &G->csr_edges[l];
    }

    const edge_type* end() const {
      if (l == r) { return 0; }
      return &G->csr_edges[r];
    }

  private:
    const Graph* G;
    int l, r;
  };

  bool is_prepared() { return prepared; }

  Graph() : N(0), M(0), prepared(0) {}
  Graph(int N) : N(N), M(0), prepared(0) {}

  void build(int n) {
    N = n, M = 0;
    prepared = 0;
    edges.clear();
    indptr.clear();
    csr_edges.clear();
    vc_deg.clear();
    vc_indeg.clear();
    vc_outdeg.clear();
  }

  void add(int frm, int to, T cost = 1, int i = -1) {
    assert(!prepared);
    assert(0 <= frm && 0 <= to && to < N);
    if (i == -1) i = M;
    auto e = edge_type({frm, to, cost, i});
    edges.eb(e);
    ++M;
  }

#ifdef FASTIO
  // wt, off
  void read_tree(bool wt = false, int off = 1) { read_graph(N - 1, wt, off); }

  void read_graph(int M, bool wt = false, int off = 1) {
    for (int m = 0; m < M; ++m) {
      INT(a, b);
      a -= off, b -= off;
      if (!wt) {
        add(a, b);
      } else {
        T c;
        read(c);
        add(a, b, c);
      }
    }
    build();
  }
#endif

  void build() {
    assert(!prepared);
    prepared = true;
    indptr.assign(N + 1, 0);
    for (auto&& e: edges) {
      indptr[e.frm + 1]++;
      if (!directed) indptr[e.to + 1]++;
    }
    for (int v = 0; v < N; ++v) { indptr[v + 1] += indptr[v]; }
    auto counter = indptr;
    csr_edges.resize(indptr.back() + 1);
    for (auto&& e: edges) {
      csr_edges[counter[e.frm]++] = e;
      if (!directed) csr_edges[counter[e.to]++] = edge_type({e.to, e.frm, e.cost, e.id});
    }
  }

  OutgoingEdges operator[](int v) const {
    assert(prepared);
    return {this, indptr[v], indptr[v + 1]};
  }

  vc<int> deg_array() {
    if (vc_deg.empty()) calc_deg();
    return vc_deg;
  }

  pair<vc<int>, vc<int>> deg_array_inout() {
    if (vc_indeg.empty()) calc_deg_inout();
    return {vc_indeg, vc_outdeg};
  }

  int deg(int v) {
    if (vc_deg.empty()) calc_deg();
    return vc_deg[v];
  }

  int in_deg(int v) {
    if (vc_indeg.empty()) calc_deg_inout();
    return vc_indeg[v];
  }

  int out_deg(int v) {
    if (vc_outdeg.empty()) calc_deg_inout();
    return vc_outdeg[v];
  }

#ifdef FASTIO
  void debug() {
    print("Graph");
    if (!prepared) {
      print("frm to cost id");
      for (auto&& e: edges) print(e.frm, e.to, e.cost, e.id);
    } else {
      print("indptr", indptr);
      print("frm to cost id");
      FOR(v, N) for (auto&& e: (*this)[v]) print(e.frm, e.to, e.cost, e.id);
    }
  }
#endif

  vc<int> new_idx;
  vc<bool> used_e;

  // G における頂点 V[i] が、新しいグラフで i になるようにする
  // {G, es}
  // sum(deg(v)) の計算量になっていて、
  // 新しいグラフの n+m より大きい可能性があるので注意
  Graph<T, directed> rearrange(vc<int> V, bool keep_eid = 0) {
    if (len(new_idx) != N) new_idx.assign(N, -1);
    int n = len(V);
    FOR(i, n) new_idx[V[i]] = i;
    Graph<T, directed> G(n);
    vc<int> history;
    FOR(i, n) {
      for (auto&& e: (*this)[V[i]]) {
        if (len(used_e) <= e.id) used_e.resize(e.id + 1);
        if (used_e[e.id]) continue;
        int a = e.frm, b = e.to;
        if (new_idx[a] != -1 && new_idx[b] != -1) {
          history.eb(e.id);
          used_e[e.id] = 1;
          int eid = (keep_eid ? e.id : -1);
          G.add(new_idx[a], new_idx[b], e.cost, eid);
        }
      }
    }
    FOR(i, n) new_idx[V[i]] = -1;
    for (auto&& eid: history) used_e[eid] = 0;
    G.build();
    return G;
  }

  Graph<T, true> to_directed_tree(int root = -1) {
    if (root == -1) root = 0;
    assert(!is_directed && prepared && M == N - 1);
    Graph<T, true> G1(N);
    vc<int> par(N, -1);
    auto dfs = [&](auto& dfs, int v) -> void {
      for (auto& e: (*this)[v]) {
        if (e.to == par[v]) continue;
        par[e.to] = v, dfs(dfs, e.to);
      }
    };
    dfs(dfs, root);
    for (auto& e: edges) {
      int a = e.frm, b = e.to;
      if (par[a] == b) swap(a, b);
      assert(par[b] == a);
      G1.add(a, b, e.cost);
    }
    G1.build();
    return G1;
  }

  HashMap<int> MP_FOR_EID;

  int get_eid(u64 a, u64 b) {
    if (len(MP_FOR_EID) == 0) {
      MP_FOR_EID.build(N - 1);
      for (auto& e: edges) {
        u64 a = e.frm, b = e.to;
        u64 k = to_eid_key(a, b);
        MP_FOR_EID[k] = e.id;
      }
    }
    return MP_FOR_EID.get(to_eid_key(a, b), -1);
  }

  u64 to_eid_key(u64 a, u64 b) {
    if (!directed && a > b) swap(a, b);
    return N * a + b;
  }

private:
  void calc_deg() {
    assert(vc_deg.empty());
    vc_deg.resize(N);
    for (auto&& e: edges) vc_deg[e.frm]++, vc_deg[e.to]++;
  }

  void calc_deg_inout() {
    assert(vc_indeg.empty());
    vc_indeg.resize(N);
    vc_outdeg.resize(N);
    for (auto&& e: edges) { vc_indeg[e.to]++, vc_outdeg[e.frm]++; }
  }
};
#line 2 "graph/find_even_cycle.hpp"

// (vs, es), size=(n+1,n)
// [R] https://scoreboard.icpc.global/46/finals46.pdf
template <typename GT>
pair<vc<int>, vc<int>> find_even_cycle(GT &G) {
  /*
  参考：https://chaoxu.prof/posts/2014-03-08-even-cycle-in-a-simple-graph.html
  DFS木を作る, 後退辺ごとにサイクルを作る
  偶サイクルができれば OK
  共通辺を持つ奇サイクルがある → それらの XOR が満たす
  どちらもない → 奇サイクルからなるカクタス → むり
  */
  static_assert(!GT::is_directed);
  const int N = G.N, M = G.M;
  if (N == 1) return {{}, {}};
  bool FOUND = 0;
  vvc<int> cyc; // edge index
  vc<int> in_cycle(M, -1);
  vc<int> path_v;
  vc<int> path_e;
  vc<bool> used(M);
  vc<bool> vis(N);
  pair<int, int> cid = {-1, -1};
  auto dfs = [&](auto &dfs, int v) -> void {
    for (auto &e: G[v]) {
      if (FOUND) return;
      if (used[e.id]) continue;
      used[e.id] = 1;

      if (!vis[e.to]) {
        vis[e.to] = 1, used[e.id] = 1;
        path_v.eb(e.to), path_e.eb(e.id);
        dfs(dfs, e.to);
        if (FOUND) return;
        POP(path_v), POP(path_e);
        continue;
      }
      // cycle 発見
      vc<int> EID = {e.id};
      int p = len(path_v) - 1;
      while (path_v[p] != e.to) { EID.eb(path_e[--p]); }
      int k = len(cyc);
      cyc.eb(EID);
      if (len(EID) % 2 == 0) {
        cid = {k, k}, FOUND = 1;
        return;
      }
      for (auto &i: EID) {
        if (in_cycle[i] != -1) {
          cid = {k, in_cycle[i]}, FOUND = 1;
          return;
        }
        in_cycle[i] = k;
      }
    }
  };
  FOR(v, N) {
    if (vis[v]) continue;
    path_v = {int(v)};
    vis[v] = 1;
    dfs(dfs, v);
  }
  if (!FOUND) return {{}, {}};
  vc<int> es = [&]() {
    if (cid.fi == cid.se) { return cyc[cid.fi]; }
    auto [a, b] = cid;
    vc<int> use(M);
    for (auto &e: cyc[a]) use[e] ^= 1;
    for (auto &e: cyc[b]) use[e] ^= 1;
    vc<int> es;
    FOR(i, M) if (use[i]) es.eb(i);
    return es;
  }();

  vc<pair<int, int>> v_to_e(N, {-1, -1});
  auto add = [&](int v, int e) -> void {
    if (v_to_e[v].fi == -1) {
      v_to_e[v].fi = e;
    } else {
      v_to_e[v].se = e;
    }
  };
  for (auto &e: es) { add(G.edges[e].frm, e), add(G.edges[e].to, e); }

  int v0 = G.edges[es[0]].frm;
  vc<int> vs = {v0};
  es.clear(), es.shrink_to_fit();
  es = {v_to_e[v0].fi};
  while (1) {
    int e = es.back();
    int a = G.edges[e].frm, b = G.edges[e].to;
    assert(vs.back() == a || vs.back() == b);
    if (vs.back() != a) swap(a, b);
    vs.eb(b);
    if (b == v0) break;
    es.eb(v_to_e[b].fi + v_to_e[b].se - e);
  }
  return {vs, es};
}

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