Java Program to Detect Cycle in a Directed Graph

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.

Java

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// 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:// www.geeksforgeeks.org/archives/18212
    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)
    {
        adj.get(source).add(dest);
    }
  
    // Returns true if the graph contains a
    // cycle, else false.
    // This function is a variation of DFS() in
    // https:// www.geeksforgeeks.org/archives/18212
    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");
        else
            System.out.println("Graph doesn't "
                               + "contain cycle");
    }
}
  
// This code is contributed by Sagar Shah.

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Output:

Graph contains cycle

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




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