class Solution {

    public ArrayList<Integer> topoSort(int V, int[][] edges) {
        ArrayList<Integer> res = new ArrayList<>();
        ArrayList<ArrayList<Integer>> adj = new ArrayList<>();
        
        for(int i=0; i<V; i++)
        {
            adj.add(new ArrayList<Integer>());
        }
        
        for(int[] edge : edges)
        {
            int u = edge[0];
            int v = edge[1];
            
            adj.get(u).add(v);
        }
        
        // Step-1 Create an indegree array
        int[] indegree = new int[V];
        for(int i=0; i<V; i++)
        {
            // if neighbour is present then increment the indegree as there is incoming edge towards it
            for(int u : adj.get(i))
            {
                indegree[u]++;
            }
        }
        
        // Step-2 Check nodes whose indegree is 0, add those nodes to Queue
        Queue<Integer> q = new LinkedList<>();
        for(int i=0; i<indegree.length; i++)
        {
            if(indegree[i] == 0)
                q.add(i);
        }
        
        // Step-3 Traverse all nodes until queue becomes empty
        while(!q.isEmpty())
        {
            int node = q.poll();
            res.add(node);
            
            // traverse through neighbour nodes and update the indegree
            for(int u : adj.get(node))
            {
                indegree[u]--;
                if(indegree[u] == 0)
                    q.add(u);
            }
        }
        return res;
    }
}