Course Schedule II

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        
        ArrayList<ArrayList<Integer>> graph = new ArrayList<>();
        int[] answer=new int[numCourses];
        int k=0;
        
        for(int i=0;i<numCourses;i++){
            graph.add(new ArrayList<Integer>());
        }
        
        for(int i=0;i<prerequisites.length;i++){
            int u=prerequisites[i][0];
            int v=prerequisites[i][1];
            graph.get(v).add(u);
        }
//         kahn's algo
        Queue<Integer> q=new LinkedList<>();
        int[] indegree = new int[numCourses];
        boolean[] visited = new boolean[numCourses];
        for(int i=0;i<numCourses;i++){
            for(int neighbour:graph.get(i)){
                indegree[neighbour]+=1;
            }
        }
        
        for(int i=0;i<numCourses;i++){
            if(indegree[i]==0){
                q.add(i);
            }
        }
        
        while(!q.isEmpty()){
            int currentVertex = q.remove();
            if(visited[currentVertex]==true)
                continue;
            
            visited[currentVertex]=true;
            answer[k]=currentVertex;
            k++;
            for(int neighbour:graph.get(currentVertex)){
                indegree[neighbour]-=1;
                if(indegree[neighbour]==0){
                    q.add(neighbour);
                }
            }
        }
        
        if(k!=numCourses){
            return new int[0];
        }   
        
        return answer;
        
        
        
    }
}