1class Solution {2    public List<List<Integer>> levelOrder(TreeNode root) {3        List<List<Integer>> result = new ArrayList<>();4        if (root == null) return result;5        Deque<TreeNode> queue = new ArrayDeque<>();6        queue.add(root);7        while (!queue.isEmpty()) {8            int size = queue.size();9            List<Integer> level = new ArrayList<>();10            for (int i = 0; i < size; i++) {11                TreeNode node = queue.remove();12                level.add(node.val);13                if (node.left != null) queue.add(node.left);14                if (node.right != null) queue.add(node.right);15            }16            result.add(level);17        }18        return result;19    }20}