Graph2/CycleDetection.hpp

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• Last update: 2023-07-17 18:02:31+09:00
• Include: #include "Graph2/CycleDetection.hpp"

Depends on

• Graph2/GraphTemplate.hpp

Verified with

• test/yosupo_CycleDetection2.test.cpp

Code

#include "./GraphTemplate.hpp"

template<typename T>
vector<int> CycleDetection(Graph<T> &g){
    int n=(int)g.size();
    vector<int> check(n,0),cyc,pre(n,-1);

    function<bool(int)> dfs=[&](int cur){
        check[cur]=1;
        for(auto &to:g[cur]){
            if(check[to]==0){
                pre[to]=cur;
                if(dfs(to)) return true;
            }else if(check[to]==1){// detect
                int v=cur;
                while(v!=to){
                    cyc.push_back(v);
                    v=pre[v];
                }
                cyc.push_back(v);
                return true;
            }
        }
        check[cur]=2;
        return false;
    };

    for(int i=0;i<n;i++){
        if(check[i]==0){
            if(dfs(i)){
                reverse(begin(cyc),end(cyc));
                return cyc;
            }
        }
    }
    return {};
}

#line 1 "Graph2/GraphTemplate.hpp"



// graph template
// ref : https://ei1333.github.io/library/graph/graph-template.hpp
template<typename T=int>
struct Edge{
    int from,to;
    T w;
    int idx;
    Edge()=default;
    Edge(int from,int to,T w=1,int idx=-1):from(from),to(to),w(w),idx(idx){}
    operator int() const{return to;}
};

template<typename T=int>
struct Graph{
    vector<vector<Edge<T>>> g;
    int V,E;
    Graph()=default;
    Graph(int n):g(n),V(n),E(0){}

    int size(){
        return (int)g.size();
    }
    void resize(int k){
        g.resize(k);
        V=k;
    }
    inline const vector<Edge<T>> &operator[](int k)const{
        return (g.at(k));
    }
    inline vector<Edge<T>> &operator[](int k){
        return (g.at(k));
    }
    void add_directed_edge(int from,int to,T cost=1){
        g[from].emplace_back(from,to,cost,E++);
    }
    void add_edge(int from,int to,T cost=1){
        g[from].emplace_back(from,to,cost,E);
        g[to].emplace_back(to,from,cost,E++);
    }
    void read(int m,int pad=-1,bool weighted=false,bool directed=false){
        for(int i=0;i<m;i++){
            int u,v;cin>>u>>v;
            u+=pad,v+=pad;
            T w=T(1);
            if(weighted) cin>>w;
            if(directed) add_directed_edge(u,v,w);
            else         add_edge(u,v,w);
        }
    }
};


#line 2 "Graph2/CycleDetection.hpp"

template<typename T>
vector<int> CycleDetection(Graph<T> &g){
    int n=(int)g.size();
    vector<int> check(n,0),cyc,pre(n,-1);

    function<bool(int)> dfs=[&](int cur){
        check[cur]=1;
        for(auto &to:g[cur]){
            if(check[to]==0){
                pre[to]=cur;
                if(dfs(to)) return true;
            }else if(check[to]==1){// detect
                int v=cur;
                while(v!=to){
                    cyc.push_back(v);
                    v=pre[v];
                }
                cyc.push_back(v);
                return true;
            }
        }
        check[cur]=2;
        return false;
    };

    for(int i=0;i<n;i++){
        if(check[i]==0){
            if(dfs(i)){
                reverse(begin(cyc),end(cyc));
                return cyc;
            }
        }
    }
    return {};
}

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