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Java Program to Detect Cycle in a Directed Graph

  • Last Updated : 02 Jan, 2019

Given a directed graph, check whether the graph contains a cycle or not. Your function should return true if the given graph contains at least one cycle, else return false. For example, the following graph contains three cycles 0->2->0, 0->1->2->0 and 3->3, so your function must return true.

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.


// A Java Program to detect cycle in a graph
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
class Graph {
    private final int V;
    private final List<List<Integer> > adj;
    public Graph(int V)
        this.V = V;
        adj = new ArrayList<>(V);
        for (int i = 0; i < V; i++)
            adj.add(new LinkedList<>());
    // This function is a variation of DFSUytil() in
    // https://
    private boolean isCyclicUtil(int i, boolean[] visited,
                                 boolean[] recStack)
        // Mark the current node as visited and
        // part of recursion stack
        if (recStack[i])
            return true;
        if (visited[i])
            return false;
        visited[i] = true;
        recStack[i] = true;
        List<Integer> children = adj.get(i);
        for (Integer c : children)
            if (isCyclicUtil(c, visited, recStack))
                return true;
        recStack[i] = false;
        return false;
    private void addEdge(int source, int dest)
    // Returns true if the graph contains a
    // cycle, else false.
    // This function is a variation of DFS() in
    // https://
    private boolean isCyclic()
        // Mark all the vertices as not visited and
        // not part of recursion stack
        boolean[] visited = new boolean[V];
        boolean[] recStack = new boolean[V];
        // Call the recursive helper function to
        // detect cycle in different DFS trees
        for (int i = 0; i < V; i++)
            if (isCyclicUtil(i, visited, recStack))
                return true;
        return false;
    // Driver code
    public static void main(String[] args)
        Graph graph = new Graph(4);
        graph.addEdge(0, 1);
        graph.addEdge(0, 2);
        graph.addEdge(1, 2);
        graph.addEdge(2, 0);
        graph.addEdge(2, 3);
        graph.addEdge(3, 3);
        if (graph.isCyclic())
            System.out.println("Graph contains cycle");
            System.out.println("Graph doesn't "
                               + "contain cycle");
// This code is contributed by Sagar Shah.
Graph contains cycle

Please refer complete article on Detect Cycle in a Directed Graph for more details!

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