102. Binary Tree Level Order Traversal

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return Collections.emptyList();
        }

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

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

        while (!queue.isEmpty()) {
            List<Integer> list = new ArrayList<>(queue.size());
            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            resultList.add(list);
        }

        return resultList;
    }
}

Reference

102. Binary Tree Level Order Traversal
剑指 Offer 32 - II. 从上到下打印二叉树 II