Create Kosaraju.java

Author: samHack03

Diff for: Kosaraju.java

@@ -0,0 +1,140 @@
+// Java program to check if a given directed graph
+// is strongly connected or not with BFS use
+import java.util.*;
+ 
+class Graph {
+  int V; // No. of vertices
+  // An array of adjacency lists
+  ArrayList<ArrayList<Integer> > adj;
+ 
+  // Constructor and Destructor
+  Graph(int V)
+  {
+    this.V = V;
+    adj = new ArrayList<>();
+    for (int i = 0; i < V; i++) {
+      adj.add(new ArrayList<>());
+    }
+  }
+ 
+  // Method to add an edge
+  void addEdge(int v, int w)
+  {
+    adj.get(v).add(w); // Add w to v’s list.
+  }
+ 
+  // A recursive function to print DFS starting from v
+  void BFSUtil(int v, boolean visited[])
+  {
+     
+    // Create a queue for BFS
+    Queue<Integer> queue = new ArrayDeque<>();
+ 
+    // Mark the current node as visited and enqueue it
+    visited[v] = true;
+    queue.add(v);
+ 
+    while (!queue.isEmpty())
+    {
+       
+      // Dequeue a vertex from queue
+      v = queue.peek();
+      queue.remove();
+ 
+      // Get all adjacent vertices of the dequeued
+      // vertex s If a adjacent has not been visited,
+      // then mark it visited and enqueue it
+      for (Integer i : adj.get(v)) {
+        if (!visited[i]) {
+          visited[i] = true;
+          queue.add(i);
+        }
+      }
+    }
+  }
+ 
+  // The main function that returns true if the
+  // graph is strongly connected, otherwise false
+  boolean isSC()
+  {
+    // Step 1: Mark all the vertices as not
+    // visited (For first BFS)
+    boolean[] visited = new boolean[V];
+    for (int i = 0; i < V; i++)
+      visited[i] = false;
+ 
+    // Step 2: Do BFS traversal starting
+    // from first vertex.
+    BFSUtil(0, visited);
+ 
+    // If BFS traversal doesn’t visit all
+    // vertices, then return false.
+    for (int i = 0; i < V; i++)
+      if (visited[i] == false)
+        return false;
+ 
+    // Step 3: Create a reversed graph
+    Graph gr = getTranspose();
+ 
+    // Step 4: Mark all the vertices as not
+    // visited (For second BFS)
+    for (int i = 0; i < V; i++)
+      visited[i] = false;
+ 
+    // Step 5: Do BFS for reversed graph
+    // starting from first vertex.
+    // Starting Vertex must be same starting
+    // point of first DFS
+    gr.BFSUtil(0, visited);
+ 
+    // If all vertices are not visited in
+    // second DFS, then return false
+    for (int i = 0; i < V; i++)
+      if (visited[i] == false)
+        return false;
+ 
+    return true;
+  }
+ 
+  // Function that returns reverse (or transpose)
+  // of this graph
+  Graph getTranspose()
+  {
+    Graph g = new Graph(V);
+    for (int v = 0; v < V; v++) {
+      // Recur for all the vertices adjacent to this
+      // vertex
+      for (Integer i : adj.get(v))
+        g.adj.get(i).add(v);
+    }
+    return g;
+  }
+}
+ 
+public class GFG {
+ 
+  // Driver program to test above functions
+  public static void main(String[] args)
+  {
+    Graph g1 = new Graph(5);
+    g1.addEdge(0, 1);
+    g1.addEdge(1, 2);
+    g1.addEdge(2, 3);
+    g1.addEdge(3, 0);
+    g1.addEdge(2, 4);
+    g1.addEdge(4, 2);
+    if (g1.isSC())
+      System.out.println("Yes");
+    else
+      System.out.println("No");
+ 
+    Graph g2 = new Graph(4);
+    g2.addEdge(0, 1);
+    g2.addEdge(1, 2);
+    g2.addEdge(2, 3);
+    if (g2.isSC())
+      System.out.println("Yes");
+    else
+      System.out.println("No");
+  }
+}