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.

public class Solution {
    public ArrayList<directedgraphnode> topSort(ArrayList<directedgraphnode> graph) {
        ArrayList<directedgraphnode> result = new ArrayList<directedgraphnode>();
        if (graph == null || graph.size() == 0) return result;
        
        HashMap<DirectedGraphNode, Integer> map = new HashMap<DirectedGraphNode, Integer>();        
        for (DirectedGraphNode node : graph) {
            for (DirectedGraphNode neighbor : node.neighbors) {
                if (map.containsKey(neighbor)) {
                    map.put(neighbor, map.get(neighbor) + 1);
                } else {
                    map.put(neighbor, 1);
                }
            }
        }
        for (DirectedGraphNode node : graph) {
            if (!map.containsKey(node)) {
                dfs(node, map, result);
            }
        }
        return result;
    }    
    public void dfs(DirectedGraphNode node,  HashMap<DirectedGraphNode, Integer> map, ArrayList<directedgraphnode> result) {
        result.add(node);
        for (DirectedGraphNode neighbor : node.neighbors) {
            map.put(neighbor, map.get(neighbor) - 1);
            if (map.get(neighbor) == 0) {
                dfs(neighbor, map, result);
            }
        }
    }
}//DFS

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> result = new ArrayList<directedgraphnode>();
        if (graph == null || graph.size() == 0) return result;
        
        HashMap<DirectedGraphNode, Integer> map = new HashMap<DirectedGraphNode, Integer>();
        
        for (DirectedGraphNode node : graph) {
            for (DirectedGraphNode neighbor : node.neighbors) {
                if (map.containsKey(neighbor)) {
                    map.put(neighbor, map.get(neighbor) + 1);
                } else {
                    map.put(neighbor, 1);
                }
            }
        }
        
        Queue<directedgraphnode> queue = new LinkedList<directedgraphnode>();
        for (DirectedGraphNode node : graph) {
            if (!map.containsKey(node)) {
                queue.add(node);
            }
        }
        while (!queue.isEmpty()) {
            DirectedGraphNode node = queue.poll();
            result.add(node);
            for (DirectedGraphNode neighbor : node.neighbors) {
                map.put(neighbor, map.get(neighbor) - 1);
                if (map.get(neighbor) == 0) {
                    queue.add(neighbor);
                }
            }
        }
        return result;
    }
}//BFS