Given an directed graph, a topological order of the graph nodes is defined as follow:

For each directed edge A -> B in graph, A must before B in the order list.
The first node in the order can be any node in the graph with no nodes direct to it.
Find any topological order for the given graph.

Notice
You can assume that there is at least one topological order in the graph.

/**
 * Definition for Directed graph.
 * class DirectedGraphNode {
 *     int label;
 *     ArrayList<DirectedGraphNode> neighbors;
 *     DirectedGraphNode(int x) { label = x; neighbors = new ArrayList<DirectedGraphNode>(); }
 * };
 */
public class Solution {
    /**
     * @param graph: A list of Directed graph node
     * @return: Any topological order for the given graph.
     */    
    public ArrayList<DirectedGraphNode> topSort(ArrayList<DirectedGraphNode> graph){
        ArrayList<DirectedGraphNode> res = new ArrayList<>();
        if (graph == null || graph.size() == 0){
            return res;
        }
       
        //計算所有節點的入度，存到HashMap indegree里面 
        Map<DirectedGraphNode,Integer> indegree = new HashMap<>();
        for (DirectedGraphNode node : graph){
            for (DirectedGraphNode neighbor : node.neighbors){
                if (indegree.containsKey(neighbor)){
                    indegree.put(neighbor, indegree.get(neighbor) + 1); 
                } else {
                    indegree.put(neighbor, 1);
                }
            }
        }

        //找到所有入度為零的點，它們都可以作為BFS的起點       
        ArrayList<DirectedGraphNode> startNodes = new ArrayList<>();
        for (DirectedGraphNode node : graph){
            if (!indegree.containsKey(node)){
                startNodes.add(node);
            }
        }
 
        //用Queue數據結構來進行BFS,每次poll()出來一個節點，它的鄰居的入度都要減一。
        //如果遇到某個鄰居入度變為0，要將改鄰居節點加入到Queue里面，并且也要加入到res里面。
        //這里不需要記錄visited與否，因為某個節點入度變為0的情況只有一次，不會重復加入。
        Queue<DirectedGraphNode> queue = new LinkedList<>();
        for (DirectedGraphNode startNode : startNodes){
            queue.offer(startNode);
            res.add(startNode);
        }
        while (!queue.isEmpty()){
            DirectedGraphNode curt = queue.poll();
            for (DirectedGraphNode neighbor : curt.neighbors){
                indegree.put(neighbor, indegree.get(neighbor) - 1);
                if (indegree.get(neighbor) == 0){
                    queue.offer(neighbor);
                    res.add(neighbor);
                }
            }
        }
        return res;
    }
}