73 lines
2.6 KiB
Java
73 lines
2.6 KiB
Java
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package com.jwetherell.algorithms.graph;
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import java.util.HashSet;
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import java.util.List;
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import java.util.Set;
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import com.jwetherell.algorithms.data_structures.Graph;
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/**
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* Cycle detection in an undirected graph using depth first search.
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*
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* @author Justin Wetherell <phishman3579@gmail.com>
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*/
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public class CycleDetection {
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private CycleDetection() { }
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/**
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* Cycle detection on a unidrected graph.
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*
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* @param graph Graph
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* @return true if a cycle exists
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*/
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public static boolean detect(Graph<Integer> graph) {
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if (graph == null)
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throw new IllegalArgumentException("Graph is NULL.");
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if (graph.getType() != Graph.TYPE.UNDIRECTED)
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throw new IllegalArgumentException("Graph is needs to be Undirected.");
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final Set<Graph.Vertex<Integer>> visitedVerticies = new HashSet<Graph.Vertex<Integer>>();
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final Set<Graph.Edge<Integer>> visitedEdges = new HashSet<Graph.Edge<Integer>>();
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final List<Graph.Vertex<Integer>> verticies = graph.getVertices();
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if (verticies == null || verticies.size() == 0)
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return false;
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// Select the zero-ith element as the root
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final Graph.Vertex<Integer> root = verticies.get(0);
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return depthFirstSearch(root, visitedVerticies, visitedEdges);
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}
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private static final boolean depthFirstSearch(Graph.Vertex<Integer> vertex, Set<Graph.Vertex<Integer>> visitedVerticies, Set<Graph.Edge<Integer>> visitedEdges) {
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if (!visitedVerticies.contains(vertex)) {
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// Found an unvisited, add to the set
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visitedVerticies.add(vertex);
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final List<Graph.Edge<Integer>> edges = vertex.getEdges();
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if (edges != null) {
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// Follow each unvisited edge, visit the vertex the edge connects to.
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for (Graph.Edge<Integer> edge : edges) {
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final Graph.Vertex<Integer> to = edge.getToVertex();
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boolean result = false;
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if (to != null && !visitedEdges.contains(edge)) {
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visitedEdges.add(edge);
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final Graph.Edge<Integer> recip = new Graph.Edge<Integer>(edge.getCost(), edge.getToVertex(), edge.getFromVertex());
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visitedEdges.add(recip);
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result = depthFirstSearch(to, visitedVerticies, visitedEdges);
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}
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if (result == true)
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return result;
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}
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}
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} else {
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// visited
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return true;
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}
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return false;
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}
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}
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