LeetCode Course Schedule II

 1 public class Solution {
 2     public int[] findOrder(int numCourses, int[][] prerequisites) {
 3         int [] res = new int[numCourses];
 4         List<Set> adjancyList = new ArrayList<Set>();
 5         for(int i = 0; i<numCourses; i++){
 6             adjancyList.add(new HashSet<Integer>());
 7         }
 8         for( int i = 0; i<prerequisites.length; i++){
 9             adjancyList.get(prerequisites[i][1]).add(prerequisites[i][0]);
10         }
11         
12         int [] countPost = new int[numCourses];
13         for(int i = 0; i<adjancyList.size(); i++){
14             Set<Integer> perSet = adjancyList.get(i);
15             Iterator<Integer> it = perSet.iterator();
16             while(it.hasNext()){
17                 countPost[it.next()]++;
18             }
19         }
20         for(int i = 0; i<numCourses; i++){
21             int k = 0;
22             while(k<countPost.length){
23                 if(countPost[k] == 0){
24                     countPost[k] = -1;
25                     break;
26                 }
27                 k++;
28             }
29             if(k == countPost.length){
30                 return new int[0];
31             }
32             res[i] = k;
33             Set<Integer> descCount = adjancyList.get(k);
34             Iterator<Integer> descIt = descCount.iterator();
35             while(descIt.hasNext()){
36                 countPost[descIt.next()]--;
37             }
38         }
39         return res;
40     }
41 }