LeetCode 210: Couse Schedule II

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        if(numCourses == 0) {
            return new int[0];
        }
        
        Map<Integer, List<Integer>> dependencies = new HashMap<>();
        int[] connections = new int[numCourses];
        for (int[] pair : prerequisites) {
            if (!dependencies.containsKey(pair[1])) {
                dependencies.put(pair[1], new ArrayList<>());
            }
            dependencies.get(pair[1]).add(pair[0]);
            connections[pair[0]]++;
        }
        
        int[] result = new int[numCourses];
        int runner = 0;
        
        Queue<Integer> queue = new LinkedList<>();
        for (int i = 0; i < numCourses; i++) {
            if (connections[i] == 0) {
                queue.offer(i);
            }
        }
        
        while (!queue.isEmpty()) {
            int parent = queue.poll();
            result[runner++] = parent;
            if (dependencies.containsKey(parent)) {
                for (int child : dependencies.get(parent)) {
                    if (connections[child]-- == 1) {
                      queue.offer(child);
                    }
                }
            }
        }
        return runner == numCourses ? result : new int[0];
    }
}