Luzhiled's Library

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:question: Strongly Connected Components(強連結成分分解)
(graph/connected-components/strongly-connected-components.hpp)

概要

与えられた有向グラフを強連結成分分解する.

グラフの任意の $2$ 頂点間に有効路が存在するとき, 有向グラフが強連結であるとよぶ. 強連結成分は, 極大で強連結な部分グラフである.

適当な頂点からDFSをして, 帰りがけ順に頂点を列挙することを, 未訪問の頂点がある間繰り返す. 次に辺をすべて逆向きにしたグラフについて, 列挙した頂点の逆順にDFS する. $1$ 回の DFS で到達できた頂点が $1$ つの強連結成分となる.

強連結成分を縮約後の頂点とそれらを結ぶ辺からなるグラフはDAGになっている.

計算量

Depends on

Required by

Verified with

Code

#pragma once

#include "../graph-template.hpp"

template< typename T = int >
struct StronglyConnectedComponents : Graph< T > {
public:
  using Graph< T >::Graph;
  using Graph< T >::g;
  vector< int > comp;
  Graph< T > dag;
  vector< vector< int > > group;

  void build() {
    rg = Graph< T >(g.size());
    for(size_t i = 0; i < g.size(); i++) {
      for(auto &e : g[i]) {
        rg.add_directed_edge(e.to, e.from, e.cost);
      }
    }
    comp.assign(g.size(), -1);
    used.assign(g.size(), 0);
    for(size_t i = 0; i < g.size(); i++) dfs(i);
    reverse(begin(order), end(order));
    int ptr = 0;
    for(int i : order) if(comp[i] == -1) rdfs(i, ptr), ptr++;
    dag = Graph< T >(ptr);
    for(size_t i = 0; i < g.size(); i++) {
      for(auto &e : g[i]) {
        int x = comp[e.from], y = comp[e.to];
        if(x == y) continue;
        dag.add_directed_edge(x, y, e.cost);
      }
    }
    group.resize(ptr);
    for(size_t i = 0; i < g.size(); i++) {
      group[comp[i]].emplace_back(i);
    }
  }

  int operator[](int k) const {
    return comp[k];
  }

private:
  vector< int > order, used;
  Graph< T > rg;

  void dfs(int idx) {
    if(exchange(used[idx], true)) return;
    for(auto &to : g[idx]) dfs(to);
    order.push_back(idx);
  }

  void rdfs(int idx, int cnt) {
    if(comp[idx] != -1) return;
    comp[idx] = cnt;
    for(auto &to : rg.g[idx]) rdfs(to, cnt);
  }
};
#line 2 "graph/connected-components/strongly-connected-components.hpp"

#line 2 "graph/graph-template.hpp"

/**
 * @brief Graph Template(グラフテンプレート)
 */
template< typename T = int >
struct Edge {
  int from, to;
  T cost;
  int idx;

  Edge() = default;

  Edge(int from, int to, T cost = 1, int idx = -1) : from(from), to(to), cost(cost), idx(idx) {}

  operator int() const { return to; }
};

template< typename T = int >
struct Graph {
  vector< vector< Edge< T > > > g;
  int es;

  Graph() = default;

  explicit Graph(int n) : g(n), es(0) {}

  size_t size() const {
    return g.size();
  }

  void add_directed_edge(int from, int to, T cost = 1) {
    g[from].emplace_back(from, to, cost, es++);
  }

  void add_edge(int from, int to, T cost = 1) {
    g[from].emplace_back(from, to, cost, es);
    g[to].emplace_back(to, from, cost, es++);
  }

  void read(int M, int padding = -1, bool weighted = false, bool directed = false) {
    for(int i = 0; i < M; i++) {
      int a, b;
      cin >> a >> b;
      a += padding;
      b += padding;
      T c = T(1);
      if(weighted) cin >> c;
      if(directed) add_directed_edge(a, b, c);
      else add_edge(a, b, c);
    }
  }

  inline vector< Edge< T > > &operator[](const int &k) {
    return g[k];
  }

  inline const vector< Edge< T > > &operator[](const int &k) const {
    return g[k];
  }
};

template< typename T = int >
using Edges = vector< Edge< T > >;
#line 4 "graph/connected-components/strongly-connected-components.hpp"

template< typename T = int >
struct StronglyConnectedComponents : Graph< T > {
public:
  using Graph< T >::Graph;
  using Graph< T >::g;
  vector< int > comp;
  Graph< T > dag;
  vector< vector< int > > group;

  void build() {
    rg = Graph< T >(g.size());
    for(size_t i = 0; i < g.size(); i++) {
      for(auto &e : g[i]) {
        rg.add_directed_edge(e.to, e.from, e.cost);
      }
    }
    comp.assign(g.size(), -1);
    used.assign(g.size(), 0);
    for(size_t i = 0; i < g.size(); i++) dfs(i);
    reverse(begin(order), end(order));
    int ptr = 0;
    for(int i : order) if(comp[i] == -1) rdfs(i, ptr), ptr++;
    dag = Graph< T >(ptr);
    for(size_t i = 0; i < g.size(); i++) {
      for(auto &e : g[i]) {
        int x = comp[e.from], y = comp[e.to];
        if(x == y) continue;
        dag.add_directed_edge(x, y, e.cost);
      }
    }
    group.resize(ptr);
    for(size_t i = 0; i < g.size(); i++) {
      group[comp[i]].emplace_back(i);
    }
  }

  int operator[](int k) const {
    return comp[k];
  }

private:
  vector< int > order, used;
  Graph< T > rg;

  void dfs(int idx) {
    if(exchange(used[idx], true)) return;
    for(auto &to : g[idx]) dfs(to);
    order.push_back(idx);
  }

  void rdfs(int idx, int cnt) {
    if(comp[idx] != -1) return;
    comp[idx] = cnt;
    for(auto &to : rg.g[idx]) rdfs(to, cnt);
  }
};
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