https://leetcode.com/problems/course-schedule-ii/

course-schedule-ii-v1.java

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<Integer>[] map = new List[numCourses];
        int[] degree = new int[numCourses];
        for (int i = 0; i < numCourses; i++) {
            map[i] = new ArrayList<Integer>();
        }
        
        for (int[] pre : prerequisites) {
            map[pre[1]].add(pre[0]);
            degree[pre[0]] += 1;
        }
        
        Queue<Integer> queue = new LinkedList();
        
        for (int i = 0; i < numCourses; i++) {
            if (degree[i] == 0) {
                queue.offer(i);
            }
        }
        
        int[] res = new int[numCourses];
        int count = 0;
        
        while(!queue.isEmpty()) {
            int curr = queue.poll();
            res[count++] = curr;
            for (int next : map[curr]) {
                degree[next] -= 1;
                if (degree[next] == 0) {
                    queue.offer(next);
                }
            }
        }
        if (count < numCourses) {
            return new int[]{};
        }
        return res;
    }
}

course-schedule-ii-v2.java

class Solution {
    
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<Integer>[] graph = new List[numCourses];
        for (int i = 0; i < numCourses; i++) {
            graph[i] = new ArrayList<>();
        }
        int[] degree = new int[numCourses];
        for (int[] pre : prerequisites) {
            graph[pre[1]].add(pre[0]);
            degree[pre[0]] += 1;
        }
        
        List<Integer> result = new ArrayList<>();
        int[] visited = new int[numCourses];
        
        for (int i = 0; i < numCourses; i++) {
            if (degree[i] != 0 || visited[i] == 2) continue;
            if (!search(result, visited, graph, i)) {
                return new int[]{};
            }
        }
        
        if (result.size() != numCourses) {
            return new int[]{};
        }
        int[] res = new int[numCourses];
        for (int i = 0; i < numCourses; i++) {
            res[i] = result.get(numCourses - 1 - i);
        }
        return res;
    }
    
    public boolean search(List<Integer> result, int[] visited, List<Integer>[] graph, int curr) {
        if (visited[curr] == 2) {
            return true;
        }
        if (visited[curr] == 1) {
            return false;
        }
        visited[curr] = 1;
        for (int next : graph[curr]) {
            if (!search(result, visited, graph, next)) {
                return false;
            }
        }
        visited[curr] = 2;
        result.add(curr);
        return true;
    }
    
    
}