lintcode-medium-Topological Sorting

/**
 * 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) {
        // write your code here
        
        if(graph == null || graph.size() == 0)
            return graph;
        
        ArrayList<DirectedGraphNode> res = new ArrayList<DirectedGraphNode>();
        HashMap<DirectedGraphNode, Integer> map = new HashMap<DirectedGraphNode, Integer>();
        
        for(DirectedGraphNode node: graph)
            map.put(node, 0);
        
        for(DirectedGraphNode node: graph){
            for(DirectedGraphNode n: node.neighbors){
                    map.put(n, map.get(n) + 1);
            }
        }
        
        DirectedGraphNode start = null;
        Queue<DirectedGraphNode> queue = new LinkedList<DirectedGraphNode>();
        
        for(DirectedGraphNode temp: graph){
            if(map.get(temp) == 0){
                start = temp;
                queue.offer(start);
                res.add(start);
            }
        }
        
        while(!queue.isEmpty()){
            DirectedGraphNode temp = queue.poll();
            
            for(DirectedGraphNode n: temp.neighbors){
                map.put(n, map.get(n) - 1);
                
                if(map.get(n) == 0){
                    queue.offer(n);
                    res.add(n);
                }
            }
        }
        
        return res;
    }
}