/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> printFromTopToBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()) {
            ArrayList<Integer> list = new ArrayList<>();
            int cnt = queue.size();
            while(cnt-- > 0) {
                TreeNode treeNode = queue.poll();
                if (treeNode == null) {
                    continue;
                }
                list.add(treeNode.val);
                queue.add(treeNode.left);
                queue.add(treeNode.right);
            }
            if (list.size() != 0) {
                res.add(list);
            }
        }
        return res;
    }
}