基于Java实现无向环和有向环的检测

package com.zhiyiyo.graph;

import com.zhiyiyo.collection.stack.LinkStack;
import com.zhiyiyo.collection.stack.Stack;

/**
 * 无向图中的环
 */
public class Cycle {
    private boolean[] marked;
    private Graph graph;
    private boolean hasCycle;

    public Cycle(Graph graph) {
        this.graph = graph;
        marked = new boolean[graph.V()];

        for (int v = 0; v < graph.V(); ++v) {
            if (!marked[v]) {
                dfs(v);
            }
        }
    }

    private void dfs(int s) {
        if (hasCycle()) return;

        Stack<Integer> vertexes = new LinkStack<>();
        vertexes.push(s);
        marked[s] = true;

        int lastVertex = s;
        while (!vertexes.isEmpty()) {
            int v = vertexes.pop();

            for (int w : graph.adj(v)) {
                if (!marked[w]) {
                    marked[w] = true;
                    vertexes.push(w);
                } else if (w != lastVertex) {
                    hasCycle = true;
                    return;
                }
            }

            lastVertex = v;
        }
    }

    /**
     * 图中是否有环
     */
    public boolean hasCycle() {
        return hasCycle;
    }
}

package com.zhiyiyo.graph;


public class LinkDigraph extends LinkGraph implements Digraph {

    public LinkDigraph(int V) {
        super(V);
    }

    @Override
    public void addEdge(int v, int w) {
        adj[v].push(w);
        E++;
    }

    @Override
    public Digraph reverse() {
        Digraph digraph = new LinkDigraph(V());
        for (int v = 0; v < V(); ++v) {
            for (int w : adj(v)) {
                digraph.addEdge(w, v);
            }
        }
        return digraph;
    }
}

package com.zhiyiyo.graph;

import com.zhiyiyo.collection.stack.LinkStack;
import com.zhiyiyo.collection.stack.Stack;

/**
 * 有向图中的环
 */
public class DirectedCycle {
    private boolean[] marked;
    private boolean[] onStack;
    private int[] edgeTo;
    private Graph graph;
    private Stack<Integer> cycle;

    public DirectedCycle(Digraph graph) {
        this.graph = graph;
        marked = new boolean[graph.V()];
        onStack = new boolean[graph.V()];
        edgeTo = new int[graph.V()];

        for (int v = 0; v < graph.V(); ++v) {
            if (!marked[v]) {
                dfs(v);
            }
        }
    }

    private void dfs(int v) {
        marked[v] = true;
        onStack[v] = true;

        for (int w : graph.adj(v)) {
            if (hasCycle()) return;
            if (!marked[w]) {
                marked[w] = true;
                edgeTo[w] = v;
                dfs(w);
            } else if (onStack[w]) {
                cycle = new LinkStack<>();
                cycle.push(w);
                for (int i = v; i != w; i = edgeTo[i]) {
                    cycle.push(i);
                }
                cycle.push(w);
            }
        }

        onStack[v] = false;
    }

    /**
     * 图中是否有环
     */
    public boolean hasCycle() {
        return cycle != null;
    }

    /**
     * 图中的一个环
     */
    public Iterable<Integer> cycle() {
        return cycle;
    }
}