剑指 Offer 32 - I. 从上到下打印二叉树

class Solution {
    public int[] levelOrder(TreeNode root) {
        if (root == null) {
            return new int[0];
        }

        List<Integer> resultList = new ArrayList<>();

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            resultList.add(node.val);
            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }
        }

        int[] resultArray = new int[resultList.size()];
        for (int i = 0; i < resultList.size(); i++) {
            resultArray[i] = resultList.get(i);
        }
        return resultArray;
    }
}

Reference