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TopologicalSortingBFS_KahnAlgorithm.java

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+package Graph;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.Scanner;
+
+public class TopologicalSortingBFS_KahnAlgorithm {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        ArrayList<ArrayList<Integer>> adjacencyList = new ArrayList<>();
+
+        // Taking input for number of nodes
+        System.out.println("Enter the number of nodes:");
+        int numberOfNodes = sc.nextInt();
+
+        // initializing adjacencyList with new ArrayList<>()
+        for(int i=1 ; i<=numberOfNodes+1 ; i++){
+            adjacencyList.add(new ArrayList<>());
+        }
+
+        // taking input of adjacent nodes of particular node
+        for(int i=1 ; i<=numberOfNodes ; i++){
+            System.out.println("Enter number of adjacent nodes to the node : " + i);
+            int countOfAdjacentNodes = sc.nextInt();
+
+            // taking input of the particular adjacent nodes to the node i
+            System.out.println("Enter adjacent nodes:");
+            for(int j=0 ; j<countOfAdjacentNodes ; j++){
+
+                // taking input of adjacent node
+                int adjacentNode = sc.nextInt();
+                // adding the adjacent node to the particular node i
+                adjacencyList.get(i).add(adjacentNode);
+            }
+        }
+
+        // This is a helper method which is used to call to topologicalSort method
+        // for printing the topological sort of the given graph
+        helperMethod(adjacencyList, numberOfNodes);
+    }
+    public static void helperMethod(ArrayList<ArrayList<Integer>> adjacencyList, int numberOfNodes){
+        int[] inDegree = new int[numberOfNodes+1];  // for storing indegree of the nodes
+        // calculating indegree of the nodes
+        for(ArrayList<Integer> adjacentNodesList : adjacencyList){
+            for(int adjacentNode : adjacentNodesList){
+                inDegree[adjacentNode]++;
+            }
+        }
+        Queue<Integer> queue = new LinkedList<>();  // for storing final topological sorting of the graph nodes
+
+        // adding the elements in to the queue data structure whose indegree is 0
+        for(int i=1 ; i<=numberOfNodes ; i++){
+            if(inDegree[i] == 0){
+                queue.offer(i);
+            }
+        }
+
+        // for storing the topological sorting of the graph
+        ArrayList<Integer> topologicalSorting = new ArrayList<>();
+
+        // While queue is not empty
+        while(!queue.isEmpty()){
+            int currentNode = queue.poll(); // current visiting node
+
+            // now check for adjacent node : decrease the indegree of the adjacent nodes of the current node
+            // if the indegree becomes 0 then add that adjacent node in to the queue data structure
+            for(int adjacentNode : adjacencyList.get(currentNode)){
+                inDegree[adjacentNode]--;   // decreasing the indegree of the adjacent node
+
+                // if the indegree of adjacent node becomes 0 then add it to the queue
+                if(inDegree[adjacentNode] == 0){
+                    queue.offer(adjacentNode);
+                }
+            }
+            topologicalSorting.add(currentNode);    // finally add the current node into the final topological sorting list
+        }
+
+        System.out.println("Topological sorting of the given graph is : " + topologicalSorting);
+    }
+}