210. Course Schedule II

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        int a[] = new int[numCourses],c=0;
        int y[] = new int[numCourses];
        int z[] = new int[0];
        int n=prerequisites.length,i;
        HashSet<Integer> l =new HashSet<Integer>();
        for(i=0;i<n;i++)
            a[prerequisites[i][0]]++;
        for(i=0;i<numCourses;i++)
            if(a[i]==0)
            {
                l.add(i);
                y[c++]=i;
            }
        int f=1,x=0;
        
        while(f==1)
        {
            f=0;
            x=0;
            for(i=0;i<n;i++)
            {
                if(prerequisites[i][0] == -1)
                    continue;
                x=1;
                if(l.contains(prerequisites[i][1]))
                {
                    if(--a[prerequisites[i][0]] == 0)
                    {
                        if(l.add(prerequisites[i][0]))
                            y[c++]=prerequisites[i][0];
                    }
                    f=1;
                    prerequisites[i][0] = -1;
                }
            }
        }
        if(x==1)
            return z;
        return y;
    }
}