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