Poison

210. Course Schedule II

发表于
BFS
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class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<List<Integer>> edgeList = new ArrayList<>(numCourses);
        for (int i = 0; i < numCourses; i++) {
            edgeList.add(new ArrayList<>());
        }

        int[] inDegrees = new int[numCourses];
        for (int[] prerequisite : prerequisites) {
            int from = prerequisite[1], to = prerequisite[0];
            inDegrees[to]++;
            edgeList.get(from).add(to);
        }

        Queue<Integer> zeroInDegreeCourseQueue = new LinkedList<>();
        for (int i = 0; i < inDegrees.length; i++) {
            if (inDegrees[i] == 0) {
                zeroInDegreeCourseQueue.add(i);
            }
        }

        List<Integer> courseList = new ArrayList<>();
        while (!zeroInDegreeCourseQueue.isEmpty()) {
            int course = zeroInDegreeCourseQueue.poll();
            courseList.add(course);

            for (Integer to : edgeList.get(course)) {
                if (--inDegrees[to] == 0) {
                    zeroInDegreeCourseQueue.add(to);
                }
            }
        }

        if (courseList.size() == numCourses) {
            return courseList.stream().mapToInt(Integer::intValue).toArray();
        } else {
            return new int[0];
        }
    }
}
DFS
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class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<List<Integer>> edgeList = new ArrayList<>(numCourses);
        for (int i = 0; i < numCourses; i++) {
            edgeList.add(new ArrayList<>());
        }

        for (int[] prerequisite : prerequisites) {
            int from = prerequisite[1], to = prerequisite[0];
            edgeList.get(from).add(to);
        }

        Stack<Integer> courseStack = new Stack<>();
        int[] visitStatuses = new int[numCourses]; // 0: not visited, 1: visiting, 2: already visited
        for (int i = 0; i < numCourses; i++) {
            if (!dfs(edgeList, courseStack, i, visitStatuses)) {
                return new int[0];
            }
        }

        int[] res = new int[courseStack.size()];
        int index = 0;
        while (!courseStack.isEmpty()) {
            res[index++] = courseStack.pop();
        }
        return res;
    }

    private boolean dfs(List<List<Integer>> edgeList, Stack<Integer> courseStack, int course, int[] visitStatuses) {
        if (visitStatuses[course] == 2) {
            return true;
        }
        if (visitStatuses[course] == 1) {
            return false;
        }

        visitStatuses[course] = 1;
        for (int to : edgeList.get(course)) {
            if (!dfs(edgeList, courseStack, to, visitStatuses)) {
                return false;
            }
        }
        visitStatuses[course] = 2;
        courseStack.push(course);

        return true;
    }
}
Reference

210. Course Schedule II