Course Schedule 2.java

import java.util.*;

class Solution {

    public int[] findOrder(int numCourses, int[][] prerequisites) {

        //A map to model the graph
        Map<Integer, List<Integer>> adjList = new HashMap<Integer, 
                                                  List<Integer>>();
        //To save the indegree of each node in the graph
        int[] indegree = new int[numCourses];
        //The output or the correct sequence of courses
        int[] topologicalOrder = new int[numCourses];

        //A for loop to create a graph with the map above by
        //making adjacency lists
        for (int i = 0; i < prerequisites.length; i++)
        {
            int dest = prerequisites[i][0];
            int src = prerequisites[i][1];
            List<Integer> lst = adjList.getOrDefault(src, 
                                           new ArrayList<Integer>());
            lst.add(dest);
            adjList.put(src, lst);

            indegree[dest] += 1;
        }

        //A Queue to save nodes with indegree == 0
        Queue<Integer> q = new LinkedList<Integer>();
        for (int i = 0; i < numCourses; i++)
        {
            if (indegree[i] == 0)
            {
                q.add(i);
            }
        }

        int i = 0;

        //poping out the indegree == 0 nodes from the graph
        //and store them in the output(topologicalOrder),
        //until the queue is empty
        while (!q.isEmpty())
        {
            int node = q.remove();
            topologicalOrder[i++] = node;

            //If the graph contains the node,
            //it updates the indegree of its neighbors
            if (adjList.containsKey(node)) {
                for (Integer neighbor : adjList.get(node))
                {
                    //decreases the indgree of the neighbor
                    indegree[neighbor]--;

                    //If the neighboring node has no
                    //other neighbors(becomes indegree == 0),
                    //adds it to the queue
                    if (indegree[neighbor] == 0)
                    {
                        q.add(neighbor);
                    }
                }
            }
        }

        //Returns the suitable order of the courses
        if (i == numCourses)
        {
            return topologicalOrder;
        }

        //If there was no suitable order, return null array
        return new int[0];
    }
}