CourseScheduleBFS.java

private class Node {
    public int val;
    public List<Node> neighbors;

    public Node(int val) {
        this.val = val;
        this.neighbors = new ArrayList<>();
    }
}

private List<Node> buildGraph(int num, int[][] prerequisites) {
    List<Node> graph = new ArrayList<>();

    for (int i = 0; i < num; i++) {
        graph.add(new Node(i));
    }

    for (int i = 0; i < prerequisites.length; i++) {
        graph.get(prerequisites[i][0]).neighbors.add(graph.get(prerequisites[i][1]));
    }

    return graph;
}

// model to a Nodes, still BFS with topological order to detect if a graph is a DAG
// https://www.geeksforgeeks.org/detect-cycle-in-a-directed-graph-using-bfs/
public boolean canFinishGraph(int numCourses, int[][] prerequisites) {
    if (numCourses <= 1 || prerequisites.length == 0 || prerequisites[0].length == 0) {
        return true;
    }
    List<Node> graph = this.buildGraph(numCourses, prerequisites);

    Queue<Node> q = new LinkedList<>();

    for (Node n : graph) {
        if (n.neighbors.size() == 0) {
            q.add(n);
        }
    }

    Set<Node> visited = new HashSet<>();
    while (!q.isEmpty()) {
        Node cur = q.poll();

        visited.add(cur);

        for (Node n : graph) {
            if (n.neighbors.contains(cur)) {
                n.neighbors.remove(cur);
                // only enqueue the nodes while there is no more neighbors
                if (n.neighbors.size() == 0) {
                    q.add(n);
                }
            }
        }

    }

    return visited.size() == numCourses;
}