Solution.java

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<Integer>[] edges = new List[numCourses];
        for (int i = 0; i < numCourses; ++i) {
            edges[i] = new ArrayList<>();
        }
        int[] indegree = new int[numCourses];
        for (int[] p : prerequisites) {
            edges[p[1]].add(p[0]);
            ++indegree[p[0]];
        }
        Queue<Integer> q = new LinkedList<>();
        for (int i = 0; i < numCourses; ++i) {
            if (indegree[i] == 0) {
                q.offer(i);
            }
        }
        int[] res = new int[numCourses];
        int cnt = 0;
        while (!q.isEmpty()) {
            int i = q.poll();
            res[cnt++] = i;
            for (int j : edges[i]) {
                --indegree[j];
                if (indegree[j] == 0) {
                    q.offer(j);
                }
            }
        }
        return cnt == numCourses ? res : new int[0];
    }
}