public boolean canFinish(int numCourses, int[][] prerequisites) {
        if(numCourses<=0)
            return false;
        //use hashmap to store adjList
        HashMap<Integer, ArrayList<Integer>> map=new HashMap<Integer, ArrayList<Integer>>();
        for(int i=0;i<prerequisites.length;i++){
            if(map.containsKey(prerequisites[i][1])){
                ArrayList<Integer> list=map.get(prerequisites[i][1]);
                list.add(prerequisites[i][0]);
                map.put(prerequisites[i][1], list);
            }
            else{
                 ArrayList<Integer> list = new ArrayList<Integer>();
                 list.add(prerequisites[i][0]);
                map.put(prerequisites[i][1],list);
            }
        }

        int[] visit=new int[numCourses];
        for(int i=0;i<numCourses;i++){
            if(!dfshelper(map,visit,i))
                return false;
        }
        return true;
    }
    public boolean dfshelper(HashMap<Integer, ArrayList<Integer>> map, int[] visit, int vertex){
        visit[vertex]=1; //visiting
        if(map.containsKey(vertex)){
            for(int u: map.get(vertex)){
                if(visit[u]==1)
                    return false;//have cycle
                if(visit[u]==0 && !dfshelper(map,visit,u))
                    return false;
            }
        }
        visit[vertex]=2;//visited
        return true;

    }