1 public class Solution {
 2     public boolean canFinish(int numCourses, int[][] prerequisites) {
 3         // init the adjacency list
 4         List<Set> posts = new ArrayList<Set>();
 5         for (int i = 0; i < numCourses; i++) {
 6             posts.add(new HashSet<Integer>());
 7         }
 8
 9         // fill the adjacency list,找到有哪几个点要依靠该点
10         for (int i = 0; i < prerequisites.length; i++) {
11             posts.get(prerequisites[i][1]).add(prerequisites[i][0]);
12         }
13
14         // count the pre-courses
15         int[] preNums = new int[numCourses];  // 计算下标的课程依赖几个课程
16         for (int i = 0; i < numCourses; i++) {
17             Set set = posts.get(i);
18             Iterator<Integer> it = set.iterator();
19             while (it.hasNext()) {
20                 preNums[it.next()]++;
21             }
22         }
23         // remove a non-pre course each time
24         for (int i = 0; i < numCourses; i++) {
25             // find a non-pre course
26             int j = 0;
27             for ( ; j < numCourses; j++) {   // 找到出度为0的点，删掉，并更新依靠该点的前驱点的个数
28                 if (preNums[j] == 0) break;
29             }
30
31             // if not find a non-pre course
32             if (j == numCourses) return false;
33
34             preNums[j] = -1;
35
36             // decrease courses that post the course
37             Set set = posts.get(j);
38             Iterator<Integer> it = set.iterator();
39             while (it.hasNext()) {
40                 preNums[it.next()]--;    // 删除依赖j节点的节点的出度
41             }
42         }
43
44         return true;
45     }
46 }