207. Course Schedule

, ,

BFS + Topological Sort

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class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[] inDegrees = new int[numCourses];
        Map<Integer, List<Integer>> nodeToNodeList = new HashMap<>();
        for (int[] prerequisite : prerequisites) {
            int from = prerequisite[1], to = prerequisite[0];
            inDegrees[to]++;
            nodeToNodeList.computeIfAbsent(from, k -> new ArrayList<>())
                    .add(to);
        }

        Queue<Integer> zeroInDegreeCourseQueue = new LinkedList<>();
        for (int i = 0; i < inDegrees.length; i++) {
            if (inDegrees[i] == 0) {
                zeroInDegreeCourseQueue.offer(i);
            }
        }
        while (!zeroInDegreeCourseQueue.isEmpty()) {
            int course = zeroInDegreeCourseQueue.poll();
            numCourses--;

            List<Integer> toList = nodeToNodeList.getOrDefault(course, Collections.emptyList());
            for (int to : toList) {
                if (--inDegrees[to] == 0) {
                    zeroInDegreeCourseQueue.offer(to);
                }
            }
        }

        return numCourses == 0;
    }
}

DFS

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class Solution {
    public boolean canFinish(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);
        }

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

        return true;
    }

    private boolean dfs(List<List<Integer>> edgeList, 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, to, visitStatuses)) {
                return false;
            }
        }
        visitStatuses[course] = 2;

        return true;
    }
}

References

207. Course Schedule
compileflow/DirectedGraph.java at v1.1.0 · alibaba/compileflow · GitHub