Luzhiled's Library

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:heavy_check_mark: Cycle Detection(閉路検出)
(graph/others/cycle-detection.hpp)

概要

有向グラフが与えられたとき, 辺素なサイクルを $1$ つみつける.

適当な頂点から DFS すると見つけられる.

使い方

計算量

$O(E + V)$

Depends on

Verified with

Code

#pragma once

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

/**
 * @brief Cycle Detection(閉路検出)
 * @docs docs/cycle-detection.md
 */
template< typename T = int >
struct CycleDetection : Graph< T > {
  using Graph< T >::Graph;
  using Graph< T >::g;

  vector< int > used;
  Edges< T > pre, cycle;

  bool dfs(int idx) {
    used[idx] = 1;
    for(auto &e : g[idx]) {
      if(used[e] == 0) {
        pre[e] = e;
        if(dfs(e)) return true;
      } else if(used[e] == 1) {
        int cur = idx;
        while(cur != e) {
          cycle.emplace_back(pre[cur]);
          cur = pre[cur].from;
        }
        cycle.emplace_back(e);
        return true;
      }
    }

    used[idx] = 2;
    return false;
  }

  Edges< T > build() {
    used.assign(g.size(), 0);
    pre.resize(g.size());
    for(int i = 0; i < (int) g.size(); i++) {
      if(used[i] == 0 && dfs(i)) {
        reverse(begin(cycle), end(cycle));
        return cycle;
      }
    }
    return {};
  }
};
#line 2 "graph/others/cycle-detection.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/others/cycle-detection.hpp"

/**
 * @brief Cycle Detection(閉路検出)
 * @docs docs/cycle-detection.md
 */
template< typename T = int >
struct CycleDetection : Graph< T > {
  using Graph< T >::Graph;
  using Graph< T >::g;

  vector< int > used;
  Edges< T > pre, cycle;

  bool dfs(int idx) {
    used[idx] = 1;
    for(auto &e : g[idx]) {
      if(used[e] == 0) {
        pre[e] = e;
        if(dfs(e)) return true;
      } else if(used[e] == 1) {
        int cur = idx;
        while(cur != e) {
          cycle.emplace_back(pre[cur]);
          cur = pre[cur].from;
        }
        cycle.emplace_back(e);
        return true;
      }
    }

    used[idx] = 2;
    return false;
  }

  Edges< T > build() {
    used.assign(g.size(), 0);
    pre.resize(g.size());
    for(int i = 0; i < (int) g.size(); i++) {
      if(used[i] == 0 && dfs(i)) {
        reverse(begin(cycle), end(cycle));
        return cycle;
      }
    }
    return {};
  }
};
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